Allow configure to work with clang and openmp disabled

The `scons` script has a [check](https://github.com/DennisOSRM/Project-OSRM/blob/master/SConstruct#L85) for clang and properly disables openmp support by avoiding trying to link the library.

But later on `omp.h` is still checked for and the configure script exits, preventing the build from continuing. This fixes this logical bug by allowing the build to continue with `clang`.