BUG: Don't set empty osx version for exteranl non-cmake projects

Building PCRE on OSX with the default configuration was failing during
configuration:

checking whether the C compiler works... no

configure: error: in
`/scratch/blowekamp/build/Slicer/PCRE-prefix/src/PCRE-build':
configure: error: C compiler cannot create executables
See `config.log' for more details.

This was due to the following flag being passed:
"-mmacosx-version-min=".

Now this flag is not set if CMAKE_OSX_DEPLOYMENT_TARGET does not have a
value.