Fix dependency generation with ccache/gcc

When running behind ccache, the output from -MMD is corrupted unless
we also use the -MF and -MT flags.

Since ccache is difficult to detect and gcc 2.x doesn't support the
-MF and -MT flags, we always use the old dependency generation method
when gcc 2.x is detected.

Originally committed as revision 19620 to svn://svn.ffmpeg.org/ffmpeg/trunk