Patch libsync.h to correct build failures in consumers, e.g. freedreno, reported by strejda. Remove platform restriction in libkms; it's needed for testing radeonkms on PPC, it could be useful on ARM, and there is no apparent downside to building it everywhere. Enable vmwgfx support in case that may be useful, which I assume was disabled only because it needed minor patching. Add a TESTS option to install the tests, which are always built anuyway, and use option helpers for MANPAGES. Disable cairo in the tests to avoid build failure due to a circular dependency as reported in PR 219420.

Poudriere 10.3/11.0 amd64/i386 and 11.0 armv6 OK, runtime verified on 11-stable amd64, ppc64, and armv6