* moves dolphin-specific settings out of Base.props
* creates exports.props for externals, allowing to easily import
individual Externals
* corrects some cruft that accumulated and probably contributed
to msbuild overbuilding
/Externals/liblzma/config.h #defines HAVE_PTHREAD_CONDATTR_SETCLOCK
which results in /Externals/liblzma/tuklib/mythread.h calling the
function pthread_condattr_setclock. This function doesn't exist in
macOS 10.12+, which are the only macOS versions Dolphin supports, but
inserting an include guard makes macOS use an alternate function.
