Fixes for libva and libdrm checks, and allow disabling build-tools subprojects
meson: Add a new option to control the source for build-tools
Similar to orc, allow distros to force bison, flex, nasm, etc, to be
provided by the system.
Needed by the Homebrew folks:
https://github.com/Homebrew/homebrew-core/pull/125996#discussion_r1166410216
meson: Fix libdrm and vaapi configure checks
We do not need fallback: for libdrm checks because the wrap file
already has a [provide] section.