Remove WINDOWS_CROSS_COMPILE

This was only used to avoid a necessary check in collab builds on
Windows. @jtanx, who does the Windows binaries, says collab currently
isn’t building on Windows anyway.
Use libossp-uuid, which works on all platforms

As a result, use UUIDs on Windows
Add ossp-uuid dependency to before_install_osx.sh

Also update an error message in fontforge_arg_with.m4
Fix include path to work irrespective of location of headers
Fix for libossp-uuid installed from source (has different name for pkgconfig)
Adjust all include of uuid.h

And reorder some so that the system-header-defeating code in ossp/uuid.h
can work.

Also ensure that the relevant -I flag is passed everywhere it is needed.
Use czmq’s own uuid library

This can either use libuuid, Window’s UuidCreate, or its own PRNG.
Merge pull request #1843 from rrthomas/master

Switch to libossp-uuid, to get UUIDs on Windows