Make compiler option conditional upon the type of compiler.

The given options are unknown to Clang.

There is also one unit test that does not compile which I have not investigated further.

To use a clang test build:

export TDAQ_RELEASE_BASE=/cvmfs/atlas-online-nightlies.cern.ch/tdaq/clang
cm_setup nightly x86_64-centos7-clang10-opt