cmake: Allow using external winmd headers to bypass download

[alvinhochun]

When integrating cppwinrt from another project, I want to vendor the winmd headers into the source tree to avoid performing the download during build. This change adds a EXTERNAL_WINMD_INCLUDE_DIR variable to enable this usage.

[sylveon]

This sounds ripe for trouble because of the pragma detect_mismatch included in the header. Every consumer is now locked in to this exact version of cppwinrt.

Edit: misread the change, thought it was for vendoring in cppwinrt.