Attempt to compile gst-ffmpeg with gcc4 (for example, with FC4Test or Devel). 
You get the following error:

if /bin/sh ../../libtool --tag=CC --mode=compile gcc -DHAVE_CONFIG_H -I. -I.
-I../.. -I../../libavcodec -DHAVE_AV_CONFIG_H=1 -fomit-frame-pointer   -msse -g
-O2 -MT libmmxsse_la-fdct_mmx.lo -MD -MP -MF ".deps/libmmxsse_la-fdct_mmx.Tpo"
-c -o libmmxsse_la-fdct_mmx.lo `test -f 'fdct_mmx.c' || echo './'`fdct_mmx.c; \
then mv -f ".deps/libmmxsse_la-fdct_mmx.Tpo" ".deps/libmmxsse_la-fdct_mmx.Plo";
else rm -f ".deps/libmmxsse_la-fdct_mmx.Tpo"; exit 1; fi
 gcc -DHAVE_CONFIG_H -I. -I. -I../.. -I../../libavcodec -DHAVE_AV_CONFIG_H=1
-fomit-frame-pointer -msse -g -O2 -MT libmmxsse_la-fdct_mmx.lo -MD -MP -MF
.deps/libmmxsse_la-fdct_mmx.Tpo -c fdct_mmx.c  -fPIC -DPIC -o
.libs/libmmxsse_la-fdct_mmx.o
In file included from fdct_mmx.c:16:
../../libavcodec/common.h:67: error: array type has incomplete element type
../../libavcodec/common.h:71: error: array type has incomplete element type
make[8]: *** [libmmxsse_la-fdct_mmx.lo] Error 1

The error apparently has to do with the AVOption struct in common.h, and the
offending struct has been removed in ffmpeg CVS as of April 3rd.  Current ffmpeg
CVS compiles fine with wtih gcc4. So updating the ffmpeg snapshot would be helpful.