Workaround for FFMS indexing bug

If FFMS_ReadIndex is used with an empty index file it gets stuck in an infinite loop instead of returning NULL
like it's supposed to do on failure. Explicitly check if the file is empty before calling it as a workaround.
Fix INSTALL in configure for Solaris systems
configure: include dependency libs in the Libs pkg-config

If only a static library is built, the user of the library that just
tries to link to the lib using the flags provided by pkg-config
might not know that only a static lib exists and that he'd have to
pass --static to pkg-config to get the internal dependencies to
be able to link the library.

For a shared build, the internal dependencies are kept in Libs.private
as before.

This matches how libav's pkg-config files are generated.
configure: don't generate a git version number if .git isn't present