[Linux] Hide manual getProcAddress from Linux

Also, changed to ARB versions, which may help compatibility on Windows.

The Linux headers already seemed to have the ARB versions available, so
hide them with #ifdef WIN32.
[Linux] Hide FSEngine on non-Windows builds

It's really only meant to handle BSAs on Windows, so hide it.
[Build] Multiplatform target / build improvements

Turn updatets into a make target like it should have been.  To update TS
files,  you must use `make/jom updatets`.

Furthermore the build steps/targets are only added if lupdate/lrelease
are found, which will help qmake fail sanely.

Also, .qm files will only be regenerated if the .ts files change or the
.qm files are not present.  It would do this every time before (e.g.
hitting F7 repeatedly in Visual Studio)