configure.ac: Use $CPPFLAGS for preprocessor flags.

diff --git a/configure.ac b/configure.ac
index c71222a93dba29cd77a109a74daed2dcd9896c3e..db5285cd99db5333e7a100e57f9d5fd4613b8c4d 100644 (file)
--- a/configure.ac
+++ b/configure.ac
@@ -26,7 +26,8 @@ AC_PROG_CC
 
 if test "x$GCC" = xyes; then
     CFLAGS="-std=c89 -pedantic -Wall -Wextra -Werror $CFLAGS"
-    CFLAGS="-D_XOPEN_SOURCE=500 -Wno-error=int-to-pointer-cast $CFLAGS"
+    CFLAGS="-Wno-error=int-to-pointer-cast $CFLAGS"
+    CPPFLAGS="-D_XOPEN_SOURCE=500 $CPPFLAGS"
     # Additional security flags.
     CFLAGS="$CFLAGS -Wformat -Wformat-security -Werror=format-security"
     CFLAGS="$CFLAGS -fstack-protector-all -Wstack-protector"