linux depend: stop using makedepend, use gcc instead

linux depend: stop using makedepend, use gcc instead

gcc can generate dependencies, so use that instead of building and
depending on the rather delicate and archaic 'makedepend'.

This fix removes makedepend from being built or used on linux, and
instead uses gcc to generate dependency data in .depend files.

I think pretty much every platform that uses gcc should do this.  I am
not sure if other compilers (intel, clang) can do this though.