Adding support for the testsuite to correctly identify the clang
compiler (previously it caught the generic GCC detection and was
identified as GCC 4.2).

This reduces some of the test failures when running the suite with
Clang - for example the macro tests no longer run or fail (since
they're only enabled for GCC).

    * compiler.c: identify the clang compiler

(do I need a separate ChangeLog line for compiler.cc too? should the
ChangeLog entry have the full path (gdb/testsuite/lib/compiler.c)?)