build: fix x264 pbuilder compilation issues

Problem is caused by uname reporting the host (not guest) architecture and
the x264 configure scripts trying to discover arch using uname.

Changed our configure call to pass in host spec from pbuilder environment
when it's available

Makefile.ffmpeg

@@ -261,6 +261,10 @@ $(LIBAVDIR)/$(LIBX264)/.tvh_build: $(LIBAVDIR)/$(LIBX264)/.tvh_download
 
 else
 
+ifneq (,$(DEB_BUILD_GNU_TYPE))
+LIBX264_HOST := --host=$(DEB_BUILD_GNU_TYPE)
+endif
+
 $(LIBAVDIR)/$(LIBX264)/.tvh_download:
 	$(call DOWNLOAD,$(LIBX264_URL),$(LIBAVDIR)/$(LIBX264_TB),$(LIBX264_SHA1))
 	#rm -rf $(LIBAVDIR)/x264-snapshot-*
@@ -280,7 +284,8 @@ $(LIBAVDIR)/$(LIBX264)/.tvh_build: \
 		--disable-lavf \
 		--disable-ffms \
 		--disable-gpac \
-		--disable-lsmash
+		--disable-lsmash \
+		$(LIBX264_HOST)
 	DESTDIR=$(LIBAVDIR)/build \
 		$(MAKE) -C $(LIBAVDIR)/$(LIBX264) install
 	@touch $@