Clang aims to be compatible with GCC when it comes to command-line
parameters. Where we enable functionality based on the presence of
a GCC-compiler, we can use the same functionality with Clang.
This patch introduces a new variable CXX_COMPAT_GCC which is true if
the compiler handles the same options as GCC, and this variable is
subsequently used in tests instead of CMAKE_COMPILER_IS_GNUCXX (which
remains if we need to test if we really have GCC, e.g. for version)
If support for OpenMP should be added, it must be specified by
setting an option. If not, then warnings for unknown pragmas are
disabled, so that the code can have #pragma omp witout getting
messages during build.
Roland Kaufmann