Put -g -O0 first as environment CFLAGS may have optimizations to apply, so should come later

Former-commit-id: a1fc5905dfc2251de417855221c82aa10e66e1d4

m4/ax_check_enable_debug.m4

@@ -76,11 +76,11 @@ AC_DEFUN([AX_CHECK_ENABLE_DEBUG],[
     AS_CASE([$enable_debug],
       [yes],[
 	AC_MSG_RESULT(yes)
-	CFLAGS="${CFLAGS} -g -O0"
-	CXXFLAGS="${CXXFLAGS} -g -O0"
-	FFLAGS="${FFLAGS} -g -O0"
-	FCFLAGS="${FCFLAGS} -g -O0"
-	OBJCFLAGS="${OBJCFLAGS} -g -O0"
+	CFLAGS="-g -O0 ${CFLAGS}"
+	CXXFLAGS="-g -O0 ${CXXFLAGS}"
+	FFLAGS="-g -O0 ${FFLAGS}"
+	FCFLAGS="-g -O0 ${FCFLAGS}"
+	OBJCFLAGS="-g -O0 ${OBJCFLAGS}"
       ],
       [info],[
 	AC_MSG_RESULT(info)