Only use Clang-specific pragma if compiler is Clang

Otherwise the compiler will probably give us a warning that these
pragmas are unknown. By default that warning is disabled with our
own build system, but we also want to be able to link to our library
without incorporating the entire build system too.
Merge pull request #385 from rolk/385_clang

Only use Clang-specific pragma if compiler is Clang
Merge branch 'master' into devel