build: allow kmscon to be cross-compiled

The genshader and genunifont utilities are run during the build process
to generate source files. In order for that to work when cross-compiling
the files need to be built using the native compiler instead of the
cross-compiler.

Add the AX_PROG_CC_FOR_BUILD m4 macro which defines various *_FOR_BUILD
variables that are the native equivalents of CC, CFLAGS, LDFLAGS, etc.
Override CC, CFLAGS and LDFLAGS for genshader and genunifont and their
object files so that they will be built natively and can be executed
during the build.

Signed-off-by: Thierry Reding <treding@nvidia.com>
(added TODO marker)
Signed-off-by: David Herrmann <dh.herrmann@gmail.com>

3 files changed, 142 insertions, 5 deletions

diff --git a/Makefile.am b/Makefile.am
index 7019290..0aeacff 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -445,8 +445,15 @@ EXTRA_DIST += $(SHADERS)
 CLEANFILES += src/static_shaders.c
 genshader_SOURCES = src/genshader.c
 
-src/static_shaders.c: $(SHADERS) genshader$(EXEEXT)
-	$(AM_V_GEN)./genshader$(EXEEXT) src/static_shaders.c $(SHADERS)
+# TODO: Using $(BUILD_EXEEXT) breaks if it doesn't equal $(EXEEXT). But stupid
+# automake doesn't allow $(EXEEXT) so lets just rely on both being the same.
+
+src/static_shaders.c: $(SHADERS) genshader$(BUILD_EXEEXT)
+	$(AM_V_GEN)./genshader$(BUILD_EXEEXT) src/static_shaders.c $(SHADERS)
+
+genshader$(BUILD_EXEEXT) $(genshader_OBJECTS): CC = $(CC_FOR_BUILD)
+genshader$(BUILD_EXEEXT) $(genshader_OBJECTS): CFLAGS = $(CFLAGS_FOR_BUILD)
+genshader$(BUILD_EXEEXT): LDFLAGS = $(LDFLAGS_FOR_BUILD)
 
 #
 # Unifont Generator
@@ -461,8 +468,12 @@ EXTRA_DIST += $(UNIFONT)
 CLEANFILES += $(UNIFONT_BIN)
 genunifont_SOURCES = src/genunifont.c
 
-$(UNIFONT_BIN): $(UNIFONT) genunifont$(EXEEXT)
-	$(AM_V_GEN)./genunifont$(EXEEXT) $(UNIFONT_BIN) $(UNIFONT)
+genunifont$(BUILD_EXEEXT) $(genunifont_OBJECTS): CC = $(CC_FOR_BUILD)
+genunifont$(BUILD_EXEEXT) $(genunifont_OBJECTS): CFLAGS = $(CFLAGS_FOR_BUILD)
+genunifont$(BUILD_EXEEXT): LDFLAGS = $(LDFLAGS_FOR_BUILD)
+
+$(UNIFONT_BIN): $(UNIFONT) genunifont$(BUILD_EXEEXT)
+	$(AM_V_GEN)./genunifont$(BUILD_EXEEXT) $(UNIFONT_BIN) $(UNIFONT)
 
 #
 # Kmscon Modules
diff --git a/configure.ac b/configure.ac
index bf3c89c..b5d9513 100644
--- a/configure.ac
+++ b/configure.ac
@@ -17,7 +17,7 @@ AC_CONFIG_HEADER(config.h)
 AC_USE_SYSTEM_EXTENSIONS
 AC_SYS_LARGEFILE
 AC_PREFIX_DEFAULT([/usr])
-AC_CANONICAL_HOST
+AC_CANONICAL_SYSTEM
 
 AM_INIT_AUTOMAKE([foreign 1.11 subdir-objects dist-xz no-dist-gzip tar-pax -Wall -Werror -Wno-portability])
 AM_SILENT_RULES([yes])
@@ -31,6 +31,7 @@ AM_SILENT_RULES([yes])
 : ${CFLAGS=""}
 
 AC_PROG_CC
+AX_PROG_CC_FOR_BUILD
 AC_PROG_CC_C99
 AM_PROG_CC_C_O
 m4_ifdef([AM_PROG_AR], [AM_PROG_AR])
diff --git a/m4/ax_prog_cc_for_build.m4 b/m4/ax_prog_cc_for_build.m4
new file mode 100644
index 0000000..6369809
--- /dev/null
+++ b/m4/ax_prog_cc_for_build.m4
@@ -0,0 +1,125 @@
+# ===========================================================================
+#   http://www.gnu.org/software/autoconf-archive/ax_prog_cc_for_build.html
+# ===========================================================================
+#
+# SYNOPSIS
+#
+#   AX_PROG_CC_FOR_BUILD
+#
+# DESCRIPTION
+#
+#   This macro searches for a C compiler that generates native executables,
+#   that is a C compiler that surely is not a cross-compiler. This can be
+#   useful if you have to generate source code at compile-time like for
+#   example GCC does.
+#
+#   The macro sets the CC_FOR_BUILD and CPP_FOR_BUILD macros to anything
+#   needed to compile or link (CC_FOR_BUILD) and preprocess (CPP_FOR_BUILD).
+#   The value of these variables can be overridden by the user by specifying
+#   a compiler with an environment variable (like you do for standard CC).
+#
+#   It also sets BUILD_EXEEXT and BUILD_OBJEXT to the executable and object
+#   file extensions for the build platform, and GCC_FOR_BUILD to `yes' if
+#   the compiler we found is GCC. All these variables but GCC_FOR_BUILD are
+#   substituted in the Makefile.
+#
+# LICENSE
+#
+#   Copyright (c) 2008 Paolo Bonzini <bonzini@gnu.org>
+#
+#   Copying and distribution of this file, with or without modification, are
+#   permitted in any medium without royalty provided the copyright notice
+#   and this notice are preserved. This file is offered as-is, without any
+#   warranty.
+
+#serial 5
+
+AU_ALIAS([AC_PROG_CC_FOR_BUILD], [AX_PROG_CC_FOR_BUILD])
+AC_DEFUN([AX_PROG_CC_FOR_BUILD], [dnl
+AC_REQUIRE([AC_PROG_CC])dnl
+AC_REQUIRE([AC_PROG_CPP])dnl
+AC_REQUIRE([AC_EXEEXT])dnl
+AC_REQUIRE([AC_CANONICAL_SYSTEM])dnl
+
+dnl Use the standard macros, but make them use other variable names
+dnl
+pushdef([ac_cv_prog_CPP], ac_cv_build_prog_CPP)dnl
+pushdef([ac_cv_prog_gcc], ac_cv_build_prog_gcc)dnl
+pushdef([ac_cv_prog_cc_works], ac_cv_build_prog_cc_works)dnl
+pushdef([ac_cv_prog_cc_cross], ac_cv_build_prog_cc_cross)dnl
+pushdef([ac_cv_prog_cc_g], ac_cv_build_prog_cc_g)dnl
+pushdef([ac_cv_exeext], ac_cv_build_exeext)dnl
+pushdef([ac_cv_objext], ac_cv_build_objext)dnl
+pushdef([ac_exeext], ac_build_exeext)dnl
+pushdef([ac_objext], ac_build_objext)dnl
+pushdef([CC], CC_FOR_BUILD)dnl
+pushdef([CPP], CPP_FOR_BUILD)dnl
+pushdef([CFLAGS], CFLAGS_FOR_BUILD)dnl
+pushdef([CPPFLAGS], CPPFLAGS_FOR_BUILD)dnl
+pushdef([LDFLAGS], LDFLAGS_FOR_BUILD)dnl
+pushdef([host], build)dnl
+pushdef([host_alias], build_alias)dnl
+pushdef([host_cpu], build_cpu)dnl
+pushdef([host_vendor], build_vendor)dnl
+pushdef([host_os], build_os)dnl
+pushdef([ac_cv_host], ac_cv_build)dnl
+pushdef([ac_cv_host_alias], ac_cv_build_alias)dnl
+pushdef([ac_cv_host_cpu], ac_cv_build_cpu)dnl
+pushdef([ac_cv_host_vendor], ac_cv_build_vendor)dnl
+pushdef([ac_cv_host_os], ac_cv_build_os)dnl
+pushdef([ac_cpp], ac_build_cpp)dnl
+pushdef([ac_compile], ac_build_compile)dnl
+pushdef([ac_link], ac_build_link)dnl
+
+save_cross_compiling=$cross_compiling
+save_ac_tool_prefix=$ac_tool_prefix
+cross_compiling=no
+ac_tool_prefix=
+
+AC_PROG_CC
+AC_PROG_CPP
+AC_EXEEXT
+
+ac_tool_prefix=$save_ac_tool_prefix
+cross_compiling=$save_cross_compiling
+
+dnl Restore the old definitions
+dnl
+popdef([ac_link])dnl
+popdef([ac_compile])dnl
+popdef([ac_cpp])dnl
+popdef([ac_cv_host_os])dnl
+popdef([ac_cv_host_vendor])dnl
+popdef([ac_cv_host_cpu])dnl
+popdef([ac_cv_host_alias])dnl
+popdef([ac_cv_host])dnl
+popdef([host_os])dnl
+popdef([host_vendor])dnl
+popdef([host_cpu])dnl
+popdef([host_alias])dnl
+popdef([host])dnl
+popdef([LDFLAGS])dnl
+popdef([CPPFLAGS])dnl
+popdef([CFLAGS])dnl
+popdef([CPP])dnl
+popdef([CC])dnl
+popdef([ac_objext])dnl
+popdef([ac_exeext])dnl
+popdef([ac_cv_objext])dnl
+popdef([ac_cv_exeext])dnl
+popdef([ac_cv_prog_cc_g])dnl
+popdef([ac_cv_prog_cc_cross])dnl
+popdef([ac_cv_prog_cc_works])dnl
+popdef([ac_cv_prog_gcc])dnl
+popdef([ac_cv_prog_CPP])dnl
+
+dnl Finally, set Makefile variables
+dnl
+BUILD_EXEEXT=$ac_build_exeext
+BUILD_OBJEXT=$ac_build_objext
+AC_SUBST(BUILD_EXEEXT)dnl
+AC_SUBST(BUILD_OBJEXT)dnl
+AC_SUBST([CFLAGS_FOR_BUILD])dnl
+AC_SUBST([CPPFLAGS_FOR_BUILD])dnl
+AC_SUBST([LDFLAGS_FOR_BUILD])dnl
+])