Add support for the clang compiler

This came from the diff at http://jogamp.org/git/?p=ant-cpptasks.git;a=commitdiff;h=2cdd1070056c0f7e9dc7f40de19c2bfdfef0432a;hp=2603158cb1a0ff29f5a23de4c0ba2ce759f574cf - it is fairly straightforward, and adds support for specifying either `clang