This patch fixes several issues found when running the `make distcheck`
target:
- In commit c561b8da, the update of gstcompat.h in Makefile.am was
forgotten.
- In commit c5756a91 add the simple_encoder_source_h in EXTRA_DIST was
forgotten.
- vpx.build.stamp is not generated at all, only vpx.configure.stamp.
- The make target distcleancheck failed because some autogenerated files
were not handled with the DISTCLEANFILES variable.
Note: `make distcheck -jXX` is not currently supported.