libvorbis: update to 1.3.7

Switched to CMake for the faster compilation and the simpler Makefile.

Minor Makefile cleanups.

Before:

time make package/libvorbis/compile -j 12
Executed in   24.40 secs   fish           external
   usr time   21.17 secs    0.00 micros   21.17 secs
   sys time    3.05 secs  426.00 micros    3.05 secs

After:

time make package/libvorbis/compile -j 12
Executed in    9.19 secs   fish           external
   usr time   11.29 secs    0.00 micros   11.29 secs
   sys time    1.43 secs  421.00 micros    1.43 secs

Signed-off-by: Rosen Penev <rosenp@gmail.com>