@xkaraman, I definitely want to keep all the `-DHAVE_XXX` flags deduced by the build system; however the compiler options like `-O0 -mtune=generic` need to be completely overridden.
I tried just defining `CFLAGS="-O2 -g …"` but that got appended/prepended (?) to the CFLAGS so RHELs `FORTIFY_SOURCE` preference didn't work - it looks like `-O0` (from current cmake) "won".
Summary: need to keep preprocessor defines, but override compiler options.