don't statically link libgcc/libstdc++ by default on OSX

A previous commit, 552b409dbc71cc9e8a0487fbd6e915f9d7330598, caused us
to always statically link for the sake of portability. However, there
currently exists a bug in (homebrew?) gcc which is causing exceptions
thrown by the application to call abort instead of properly unwinding
and allowing the exception to be caught. An example can be found here:
https://bugs.busybox.net/show_bug.cgi?id=7208

HHVM uses throw/catch in Repo::schemaExists to determine if a table
exists. This caused a runtime crash on OSX.

Ideally we'll be moving to clang on OSX in the near future anyway, but
if, for whatever reason, we want to use gcc *and* statically link at
some point, we can use the recently added cmake option STATIC_CXX_LIB
to do so.