Log Message:
emulators/openmsx: Fix linkpkgsrc circular reference

Probably this was a problem for non-gcc / non-clang compilers, but such
a compiler wasn't tested.  When lintpkgsrc tried to scan this package,
it spun around forever.  Rewrite the CXX override code so this doesn't
happen.  Fixes lintpkgsrc scan and probably build for rarer compilers
as well.