Fix unix/Makefile to build on OSX

Please comment and review - I am not an expert in the subtleties of Makefiles!

This unix/Makefile change should allow the unix port to build on OSX without breaking stmhal build on OSX or affecting other platforms. Tested build for unix and stmhal on OSX 10.7.5 (Lion):

Force OSX to compile with clang even if gcc is available
Change LDFLAGS syntax to be compatible with clang
Fix questionable syntax on line 90
Remove extraneous tab character on line 102