gcc49-libs: Make this function correctly on Darwin.

Firstly we need to modify the copied libraries so that their install_name
matches the new location, otherwise check-shlibs thinks that they still
come from the main gcc49 package.  Secondly we need to avoid the stub
libgcc_ext libraries that are used by default as we are unable to modify
their install_name, just link directly to libgcc.  Bump PKGREVISION.

(jperkin)