Improve version.h generation

Improve version.h generation

Force version.sh to run whenever the version might have changed,
regardless of what is being built.  This is done by attaching the
dependencies to a dummy file (.version) which is included from the
makefile.  As make will always attempt to rebuild any included files
before considering other rules, this ensures that the real version.h
is (re-)created before it is required by any source file.

Originally committed as revision 22420 to svn://svn.ffmpeg.org/ffmpeg/trunk