GROMACS CPU detection fails for v2019 to 2019.3 with CMake from 2019b toolchain and newer

[akesandgren]

The CPU detection code in GROMACS 2019 to 2019.3 does not work correctly with CMake 3.15.3 at least.
Result is:

-- Did not detect build CPU vendor - detection program did not compile
-- Disabling RDTSCP support - not detected on the build host

[akesandgren]

The simple fix is done in easybuilders/easybuild-easyconfigs#14212

[akesandgren]

This problem is now detected by #2609

[boegel]

Closing this as both #2609 and easybuilders/easybuild-easyconfigs#14212 are merged