build: refactor compiler_warnings.m4 macro

NM_COMPILER_WARNINGS still works the same, but
rename variables to have a CFLAGS_* prefix.

Also cleanup the construction of CFLAGS by appending
to CFLAGS_MORE_WARNINGS variable instead of appending
to CFLAGS, and resetting to SAVE_CFLAGS.

Signed-off-by: Thomas Haller <thaller@redhat.com>

m4/compiler_warnings.m4

@@ -5,24 +5,22 @@ AC_DEFUN([NM_COMPILER_WARNINGS],
 AC_MSG_CHECKING(for more warnings)
 if test "$GCC" = "yes" -a "$set_more_warnings" != "no"; then
 	AC_MSG_RESULT(yes)
-	CFLAGS="-Wall -std=gnu89 $CFLAGS"
+	CFLAGS_SAVED="$CFLAGS"
+	CFLAGS_MORE_WARNINGS="-Wall -std=gnu89"
 
 	dnl clang only warns about unknown warnings, unless
 	dnl called with "-Werror=unknown-warning-option"
 	dnl Test if the compiler supports that, and if it does
 	dnl attach it to the CFLAGS.
-	SAVE_CFLAGS="$CFLAGS"
-	EXTRA_CFLAGS="-Werror=unknown-warning-option"
-	CFLAGS="$SAVE_CFLAGS $EXTRA_CFLAGS"
+	CFLAGS_EXTRA="-Werror=unknown-warning-option"
+	CFLAGS="$CFLAGS_MORE_WARNINGS $CFLAGS_EXTRA $CFLAGS_SAVED"
 	AC_TRY_COMPILE([], [],
 		has_option=yes,
 		has_option=no,)
 	if test $has_option = no; then
-		EXTRA_CFLAGS=
+		CFLAGS_EXTRA=
 	fi
-	CFLAGS="$SAVE_CFLAGS"
 	unset has_option
-	unset SAVE_CFLAGS
 
 	for option in -Wshadow -Wmissing-declarations -Wmissing-prototypes \
 		      -Wdeclaration-after-statement -Wformat-security \
@@ -31,26 +29,24 @@ if test "$GCC" = "yes" -a "$set_more_warnings" != "no"; then
 		      -Wundef -Wimplicit-function-declaration \
 		      -Wpointer-arith -Winit-self \
 		      -Wmissing-include-dirs -Waggregate-return; do
-		SAVE_CFLAGS="$CFLAGS"
-		CFLAGS="$CFLAGS $EXTRA_CFLAGS $option"
+		CFLAGS="$CFLAGS_MORE_WARNINGS $CFLAGS_EXTRA $option $CFLAGS_SAVED"
 		AC_MSG_CHECKING([whether gcc understands $option])
 		AC_TRY_COMPILE([], [],
 			has_option=yes,
 			has_option=no,)
-		if test $has_option = no; then
-			CFLAGS="$SAVE_CFLAGS"
-		else
-			CFLAGS="$SAVE_CFLAGS $option"
+		if test $has_option != no; then
+			CFLAGS_MORE_WARNINGS="$CFLAGS_MORE_WARNINGS $option"
 		fi
 		AC_MSG_RESULT($has_option)
 		unset has_option
-		unset SAVE_CFLAGS
 	done
 	unset option
-	unset EXTRA_CFLAGS
+	unset CFLAGS_EXTRA
 	if test "x$set_more_warnings" = xerror; then
-		CFLAGS="$CFLAGS -Werror"
+		CFLAGS_MORE_WARNINGS="$CFLAGS_MORE_WARNINGS -Werror"
 	fi
+	CFLAGS="$CFLAGS_MORE_WARNINGS $CFLAGS_SAVED"
+	unset CFLAGS_SAVED
 else
 	AC_MSG_RESULT(no)
 fi