Allow linking 64-bit executables on Solaris-derived i86 platforms

On OpenIndiana (and as far as I can tell on other Solaris-derived platforms (OmniOS, Solaris 11)) to link 64-bit executable linker should receive '-m64' option. Also usually 64-bit libraries live in separate prefix (/usr/lib/amd64 or /usr/g++/amd64)