When configuring a cmake project with cmake 3.10.1 using the XL C and C++ compilers, cmake identifies the compiler as clang/clang++: $ CC=xlc cmake .. -- The C compiler identification is Clang 4.0.1 [...] -- Check for working C compiler: /usr/bin/xlc -- Check for working C compiler: /usr/bin/xlc -- works [...] $ xlc --version IBM XL C/C++ for Linux, V13.1.7 (Beta 2) Version: 13.01.0007.0000 Due to this misidentification, cmake is also not able to find the correct options to enable OpenMP etc.