math/the-algorithms-c++: fix build without libomp

CMake Error: CMAKE_C_COMPILER not set, after EnableLanguage
CMake Error: CMAKE_CXX_COMPILER not set, after EnableLanguage
CMake Error at /usr/local/share/cmake/Modules/FindOpenMP.cmake:261 (try_compile):

Failed to configure test project build system.

Call Stack (most recent call first):

/usr/local/share/cmake/Modules/FindOpenMP.cmake:537 (_OPENMP_GET_FLAGS)
CMakeLists.txt:21 (find_package)