build-sys: fix intention behind OPENMP_FLAGS
build-sys: fix intention behind OPENMP_FLAGS

Defining CC would override (e.g. for Clang).

piotr1212/carbon-c-relay,lyft/carbon-c-relay,lyft/carbon-c-relay,piotr1212/carbon-c-relay,grobian/carbon-c-relay,piotr1212/carbon-c-relay,tjamesturner/carbon-c-relay,grobian/carbon-c-relay,rtoma/carbon-c-relay,Skyscanner/carbon-c-relay,rtoma/carbon-c-relay,zzl0/carbon-c-relay,lyft/carbon-c-relay,Skyscanner/carbon-c-relay,grobian/carbon-c-relay,tjamesturner/carbon-c-relay,piotr1212/carbon-c-relay,zzl0/carbon-c-relay