Improve check for $VARNISHSRC

configure.ac

@@ -27,6 +27,13 @@ if test "x$VARNISHSRC" = x; then
 fi
 VARNISHAPI_CFLAGS="$VARNISHAPI_CFLAGS -I$VARNISHSRC/bin/varnishd"
 
+OCFLAGS="$CFLAGS"
+CFLAGS="$VARNISHAPI_CFLAGS"
+AC_CHECK_HEADERS([cache/cache_obj.h], [],
+	[AC_MSG_ERROR([Missing \$VARNISHSRC development files.])],
+	[[#include <cache/cache.h>]])
+CFLAGS="$OCFLAGS"
+
 AC_PATH_TOOL(VMOD_BLOB, [libvmod_blob.so], [],
 	[${VARNISHAPI_VMODDIR}:${vmoddir}])
 if test "x$VMOD_BLOB" = x ; then
@@ -67,6 +74,7 @@ AS_ECHO("
 
 	compiler:     $CC
 	cflags:       $CFLAGS
+	vapi cflags:  $VARNISHAPI_CFLAGS
 	ldflags:      $LDFLAGS
 
 	blob:         $VMOD_BLOB