[libvirt] [PATCH] Improve compiler flag checking

[john.levon at sun.com]

# HG changeset patch
# User john.levon at sun.com
# Date 1229399267 28800
# Node ID 1f7707a3d49b20397011897cdc04c347322ad0c5
# Parent  a6c317b5fe1b4081ccb40ba90af7d31298cf6b44
Improve compiler flag checking

Some compilers (including GCC) don't set the return value consistently
if an erroneous option is passed on the command line. Account for that.

Signed-off-by: John Levon <john.levon at sun.com>

diff --git a/acinclude.m4 b/acinclude.m4
--- a/acinclude.m4
+++ b/acinclude.m4
@@ -45,21 +45,9 @@ AC_DEFUN([LIBVIRT_COMPILE_WARNINGS],[
 	;;
     esac
 
-    compiler_flags=
+    COMPILER_FLAGS=
     for option in $try_compiler_flags; do
-	SAVE_CFLAGS="$CFLAGS"
-	CFLAGS="$CFLAGS $option"
-	AC_MSG_CHECKING([whether gcc understands $option])
-	AC_TRY_LINK([], [],
-		has_option=yes,
-		has_option=no,)
-	CFLAGS="$SAVE_CFLAGS"
-	AC_MSG_RESULT($has_option)
-	if test $has_option = yes; then
-	  compiler_flags="$compiler_flags $option"
-	fi
-	unset has_option
-	unset SAVE_CFLAGS
+        gl_COMPILER_FLAGS($option)
     done
     unset option
     unset try_compiler_flags
@@ -85,7 +73,7 @@ AC_DEFUN([LIBVIRT_COMPILE_WARNINGS],[
     fi
     AC_MSG_RESULT($complCFLAGS)
 
-    WARN_CFLAGS="$compiler_flags $complCFLAGS"
+    WARN_CFLAGS="$COMPILER_FLAGS $complCFLAGS"
     AC_SUBST(WARN_CFLAGS)
 ])
 
diff --git a/m4/compiler-flags.m4 b/m4/compiler-flags.m4
--- a/m4/compiler-flags.m4
+++ b/m4/compiler-flags.m4
@@ -23,12 +23,14 @@ AC_DEFUN([gl_COMPILER_FLAGS],
 AC_DEFUN([gl_COMPILER_FLAGS],
   [AC_MSG_CHECKING(whether compiler accepts $1)
    AC_SUBST(COMPILER_FLAGS)
-   ac_save_CFLAGS="$CFLAGS"
-   CFLAGS="$CFLAGS $1"
-   AC_TRY_COMPILE(,
-    [int x;],
-    COMPILER_FLAGS="$COMPILER_FLAGS $1"
-    AC_MSG_RESULT(yes),
-    AC_MSG_RESULT(no))
-  CFLAGS="$ac_save_CFLAGS"
+   echo 'int x;' >conftest.c
+   $CC $CFLAGS $1 -c conftest.c 2>conftest.err
+   ret=$?
+   if test $ret != 0 -o -s conftest.err; then
+       AC_MSG_RESULT(no)
+   else
+       AC_MSG_RESULT(yes)
+       COMPILER_FLAGS="$COMPILER_FLAGS $1"
+   fi
+   rm -f conftest*
  ])