GppLinker: disable unsupported flags for clang

OS X 10.9 Mavericks now uses LLVM with clang by default. While it
pretends to be gcc, there are a few incompatibilities. In particular,
the -static-libgcc and -shared-libgcc flags are unsupported, and
-lstdc++-static does not work because that static library is not
present on standard include paths.

[JES: rebased on top of the previous clang work]

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>