Add option to compile with clang in Geant4 port

[lpgaff (Liam Gaffney)]

Having installed Geant4 using Macports, I cannot run any compiled programs due to the mismatch between libraries built with clang and those built with gcc. It turns out that cmake is finding gcc when Geant4 is built, but everything else is built with clang.

I have hacked this for now by adding

-DCMAKE_C_COMPILER=/opt/local/bin/clang -DCMAKE_CXX_COMPILER=/opt/local/bin/clang++

to the configure.args-append options of the Portfile so that it now reads:

configure.args-append -DGEANT4_INSTALL_DATA=OFF \
                     -DCMAKE_INSTALL_BINDIR=${prefix}/libexec/Geant4/Geant${geant.version_simple} \
                     -DCMAKE_INSTALL_DATAROOTDIR=${prefix}/share/Geant4 \
                     -DCMAKE_INSTALL_INCLUDEDIR=${prefix}/include/Geant4/Geant${geant.version_simple} \
                     -DCMAKE_INSTALL_LIBDIR=${prefix}/lib/Geant4/Geant${geant.version_simple} \
                     -DCMAKE_INSTALL_NAME_DIR=${prefix}/lib/Geant4/Geant${geant.version_simple} \
                     -DCMAKE_INSTALL_RPATH=${prefix}/lib/Geant4/Geant${geant.version_simple} \
                     -DCMAKE_C_COMPILER=/opt/local/bin/clang -DCMAKE_CXX_COMPILER=/opt/local/bin/clang++

Of course, it would be nice if this was done properly as an option via the "variants" method. Can somebody please advise here?

Cheers, Liam