add easyblock for HPCG

simply setting `$CXX` in the environment doesn't mean the `Makefile` will pick it up

passing it on the `make` command line like this overrides the value set in the `Makefile` however (which is exactly what's needed here)

w.r.t. `$CXXFLAGS`: we want to inject our own `$CXXFLAGS`, but shouldn't destroy the extra stuff that's internally used in the `Makefile`, including the `$(HPCG_DEFS)`