Add a pkgconfig file; Adapted from patch by Stefan Kost.

NB. this branch has no Version_script, no docs and no Win32 directory,
so r17149,r17151,r17152 cannot be applied.

git-svn-id: https://svn.xiph.org/branches/lowmem-no-byte/Tremor@17155 0101bb08-14d6-0310-b084-bc0e0c8e3800