separate CFLAGS that don't work in C++

Index: configure.ac
--- configure.ac.orig
+++ configure.ac
@@ -433,6 +433,7 @@ AM_CONDITIONAL([WINDOWS_BUILD],
 
 dnl Final substitution
 AC_SUBST([GENERAL_CFLAGS])
+AC_SUBST([ONLY_CFLAGS])
 AC_SUBST([IL_CFLAGS])
 AC_SUBST([IL_LIBS])
 AC_SUBST([ILU_CFLAGS])