- downgrades some diagnostics about nonconformant code from errors to
warnings. Oddly enough, the errors actually arise from STL library
elements shipped with gcc itself. Affects kahip compilation with
gcc-6, gcc-7
- required if there is no system openmp and libomp or libgomp are
only found in the clang hierarchy
STYLE: add some notes in the openmp rules.
- the _OPENMP macro is now used in low-level testing files
