Prepare for powerpc-on-clang by deleting hard-coded tests for libstdc++.so
as a stand-in for "are we running on gcc".

These 4 cases are outliers because they set compiler:nestedfct which
currently hardwires gcc. So the real test for including -fnested-functions
into CFLAGS is "are we on the ancient base gcc".

Approved by: portmgr (tier-2 blanket)