[PATCH 1/2] configure.ac: add new autoconf macro for checking valid compiler flags

[Elia Pinto <gitter.spiros@gmail.com>]

Add GIT_CC_CHECK_FLAG_APPEND, GIT_CC_CHECK_FLAGS_APPEND
and GIT_CC_CHECK_LDFLAGS autoconf macro.

Signed-off-by: Elia Pinto <gitter.spiros@gmail.com>
---
 Makefile     |    6 ++++--
 configure.ac |   45 +++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 49 insertions(+), 2 deletions(-)

diff --git a/Makefile b/Makefile
index 827006b..23485f1 100644
--- a/Makefile
+++ b/Makefile
@@ -344,10 +344,12 @@ GIT-VERSION-FILE: FORCE
 	@$(SHELL_PATH) ./GIT-VERSION-GEN
 -include GIT-VERSION-FILE
 
+GIT_CFLAGS  =
+GIT_LDFLAGS =
 # CFLAGS and LDFLAGS are for the users to override from the command line.
 
-CFLAGS = -g -O2 -Wall
-LDFLAGS =
+CFLAGS = -g -O2 -Wall $(GIT_CFLAGS)
+LDFLAGS = $(GIT_LDFLAGS)
 ALL_CFLAGS = $(CPPFLAGS) $(CFLAGS)
 ALL_LDFLAGS = $(LDFLAGS)
 STRIP ?= strip
diff --git a/configure.ac b/configure.ac
index 6af9647..c67ca60 100644
--- a/configure.ac
+++ b/configure.ac
@@ -139,6 +139,51 @@ if test -n "$1"; then
 fi
 ])
 
+
+dnl Check if FLAG in ENV-VAR is supported by compiler and append it
+dnl to WHERE-TO-APPEND variable
+dnl GIT_CC_CHECK_FLAG_APPEND([WHERE-TO-APPEND], [ENV-VAR], [FLAG])
+
+AC_DEFUN([GIT_CC_CHECK_FLAG_APPEND], [
+  AC_CACHE_CHECK([if $CC supports flag $3 in envvar $2],
+                 AS_TR_SH([cc_cv_$2_$3]),
+          [eval "AS_TR_SH([cc_save_$2])='${$2}'"
+           eval "AS_TR_SH([$2])='-Werror $3'"
+           AC_LINK_IFELSE([AC_LANG_SOURCE([int a = 0; int main(void) { return a; } ])],
+                          [eval "AS_TR_SH([cc_cv_$2_$3])='yes'"],
+                          [eval "AS_TR_SH([cc_cv_$2_$3])='no'"])
+           eval "AS_TR_SH([$2])='$cc_save_$2'"])
+
+  AS_IF([eval test x$]AS_TR_SH([cc_cv_$2_$3])[ = xyes],
+        [eval "$1='${$1} $3'"])
+])
+
+dnl GIT_CC_CHECK_FLAGS_APPEND([WHERE-TO-APPEND], [ENV-VAR], [FLAG1 FLAG2])
+AC_DEFUN([GIT_CC_CHECK_FLAGS_APPEND], [
+  for flag in $3; do
+    GIT_CC_CHECK_FLAG_APPEND($1, $2, $flag)
+  done
+])
+
+dnl Check if the flag is supported by linker (cacheable)
+dnl GIT_CC_CHECK_LDFLAGS([FLAG], [ACTION-IF-FOUND],[ACTION-IF-NOT-FOUND])
+
+AC_DEFUN([GIT_CC_CHECK_LDFLAGS], [
+  AC_CACHE_CHECK([if $CC supports $1 flag],
+    AS_TR_SH([cc_cv_ldflags_$1]),
+    [ac_save_LDFLAGS="$LDFLAGS"
+     LDFLAGS="$LDFLAGS $1"
+     AC_LINK_IFELSE([int main() { return 1; }],
+       [eval "AS_TR_SH([cc_cv_ldflags_$1])='yes'"],
+       [eval "AS_TR_SH([cc_cv_ldflags_$1])="])
+     LDFLAGS="$ac_save_LDFLAGS"
+    ])
+
+  AS_IF([eval test x$]AS_TR_SH([cc_cv_ldflags_$1])[ = xyes],
+    [$2], [$3])
+])
+
+
 ## Configure body starts here.
 
 AC_PREREQ(2.59)
-- 
1.7.10.4