meson: Add options to control optional parts

Distributions want explicit control over optional parts so they can
state runtime dependencies before building. Let's restore the
functionality autotools used to provide.

Where possible, the selection is done by choosing whether to build a
particular item and the option name is build_$item. Example:
build_overlay. Where not possible, the option name is
with_$item. Example: with_valgrind.

Array options require a bump of required meson version to 0.44. Debian
stable has meson 0.37 which is already too old, stable-backports has
0.45, CI uses 0.45. Mesa's meson requirement is 0.44.1, for a
perspective.

Note, the old hack for not building docs when cross-compiling is
gone, as doc building can be explicitly controlled now.

v2: glib not optional
v3: bump meson version to 0.44

Signed-off-by: Petri Latvala <petri.latvala@intel.com>
Cc: Matt Turner <mattst88@gmail.com>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Eric Anholt <eric@anholt.net>
Cc: Arkadiusz Hiler <arkadiusz.hiler@intel.com>
Signed-off-by: Arkadiusz Hiler <arkadiusz.hiler@intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>