meson: Add build Intel "anv" vulkan driver

meson: Add build Intel "anv" vulkan driver

This allows building and installing the Intel "anv" Vulkan driver using
meson and ninja, the driver has been tested against the CTS and has
seems to pass the same series of tests (they both segfault when the CTS
tries to run wayland wsi tests).

There are still a mess of TODO, XXX, and FIXME comments in here. Those
are mostly for meson bugs I'm trying to fix, or for additional things to
implement for other drivers/features.

I have configured all intermediate libraries and optional tools to not
build by default, meaning they will only be built if they're pulled in
as a dependency of a target that will actually be installed) this allows
us to avoid massive if chains, while ensuring that only the bits that
need to be built are.

v2: - enable anv, x11, and wayland by default
    - add configure option to disable valgrind
v3: - fix typo in meson_options (Nicholas)
v4: - Remove dead code (Eric)
    - Remove change to generator that was from v0 (Eric)
    - replace if chain with loop (Eric)
    - Fix typos (Eric)
    - define HAVE_DLOPEN for both libdl and builtin dl cases (Eric)
v5: - rebase on util string buffer implementation

Signed-off-by: Dylan Baker <dylanx.c.baker@intel.com>
Reviewed-by: Eric Anholt <eric@anholt.net> (v4)