distcheck: use the original CFLAGS for the inner configure

The previous commit did not solve the issue for all cases
- we also change OFLAGS in configure

Makefile.am

@@ -22,7 +22,7 @@ DISTCHECK_CONFIGURE_FLAGS = \
     --enable-developer-warnings \
     --enable-debugging-symbols \
     --enable-dependency-tracking \
-    CFLAGS="$(OCFLAGS)"
+    CFLAGS="$(EXTCFLAGS)"
 
 install-data-local:
 	$(install_sh) -d -m 0755 $(DESTDIR)$(localstatedir)/varnish

configure.ac

@@ -10,7 +10,10 @@ AC_CONFIG_AUX_DIR([build-aux])
 AC_USE_SYSTEM_EXTENSIONS
 
 # save command line CFLAGS for use in VCC_CC (to pass through things like -m64)
+# and make distcheck configure
 OCFLAGS="$CFLAGS"
+EXTCFLAGS="$CFLAGS"
+AC_SUBST(EXTCFLAGS)
 
 AC_CANONICAL_SYSTEM
 AC_LANG(C)