contribs: fix error handling while compressing tar archives

When the shell evaluates 'foo | bar', it returns the status of bar, not
foo. 'foo' can fail silently: so long as 'bar' succeeds, make will
not notice and will therefore not delete the target.

So lets use 'tar cv(z|j|J)' instead of 'tar cv | (gzip|bzip2|xz)'.

contrib/src/ffmpeg/rules.mak

@@ -122,7 +122,7 @@ ffmpeg-$(FFMPEG_VERSION).tar.gz:
 
 $(TARBALLS)/ffmpeg-svn.tar.gz:
 	$(SVN) export $(FFMPEG_SVN) ffmpeg-svn
-	tar cv ffmpeg-svn | gzip > $@
+	tar cvz ffmpeg-svn > $@
 
 FFMPEG_VERSION := svn
 

contrib/src/tremor/rules.mak

@@ -7,7 +7,7 @@ endif
 $(TARBALLS)/tremor-svn.tar.xz:
 	rm -Rf tremor-svn
 	$(SVN) export http://svn.xiph.org/trunk/Tremor tremor-svn
-	tar cv tremor-svn | xz > $@
+	tar cvJ tremor-svn > $@
 
 .sum-tremor: tremor-svn.tar.xz
 	$(warning Integrity check skipped.)