rtmpdump: fix CFLAGS/LDFLAGS

The rtmpdump makefile looks at XCFLAGS/XLDFLAGS for additional flags,
so use those instead of completely overriding the flags.

Fixes http://autobuild.buildroot.net/results/190/1902a6613e5fc9656842fbcbd5731ac871d7ef59/

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/rtmpdump/rtmpdump.mk

@@ -33,8 +33,8 @@ endif
 define RTMPDUMP_BUILD_CMDS
 	$(MAKE) CRYPTO=$(RTMPDUMP_CRYPTO) \
 		prefix=/usr \
-		$(TARGET_CONFIGURE_OPTS) \
-		CFLAGS="$(RTMPDUMP_CFLAGS)" \
+		XCFLAGS="$(RTMPDUMP_CFLAGS)" \
+		XLDFLAGS="$(TARGET_CFLAGS)" \
 		CROSS_COMPILE="$(TARGET_CROSS)" \
 		-C $(@D)/librtmp
 endef