cxx-openmp-ignition test fails if OpenMP is not available

[speth]

Problem description

Building the latest version of Cantera (commit ) on Mac OS X Catalina from source, the test cxx-openmp-ignition fails as the build cannot locate the "omp.h" file.

Installing libomp via Homebrew seems to solve the problem.

Presumably, we should skip this test if OpenMP is not available.

Steps to reproduce

1. Compile Cantera on a system where omp.h is not present.
2. run scons test

System information

• Cantera version: current main (2cd0619)
• OS: macOS Catalina

Additional context

This issue was originally reported by @santoshshanbhogue on the Cantera Users' Group (https://groups.google.com/g/cantera-users/c/zefZRgeKUbw)