Add proper dependencies for the command line binaries

This makes sure that all the dependency libs are built before trying
to link them together, fixing parallel make (which occasionally
failed due to trying to link the encoder or decoder command line
binaries before the libs had been built) and also fixes building an
individual component by e.g. "make h264enc".
Merge pull request #116 from mstorsjo/make-deps

Add proper dependencies for the command line binaries