distcheck: use the original CFLAGS for the inner configure

In particular with --enable-developer-warnings, we select strict compiler
options for CFLAGS, which are too strict for the configure built-in conftest.c
code.

Make distcheck calls configure again for an out-of-tree build, so it would
see the CFLAGS the "outer" configure built. To avoid this situation, we
restore the original CFLAGS for the inner configure.

I wonder if there is a better, more autoconf-ish, solution to this problem.

Makefile.am

@@ -21,7 +21,8 @@ EXTRA_DIST = \
 DISTCHECK_CONFIGURE_FLAGS = \
     --enable-developer-warnings \
     --enable-debugging-symbols \
-    --enable-dependency-tracking
+    --enable-dependency-tracking \
+    CFLAGS="$(OCFLAGS)"
 
 install-data-local:
 	$(install_sh) -d -m 0755 $(DESTDIR)$(localstatedir)/varnish