by leaving out the choices keyword.

Add an array type to user options

One thing that makes cross compiling with meson a pain is the need for
cross files. The problem is not with cross files themselves (they're
actually rather brilliant in that they allow for a much greater deal of
flexibility than autotools hardcoded paths approach) but that each user
needs to reimplement them themselves, when for most people what they
really want is a cross file that could be provided by their distro, all
they really want is the correct toolchain.

This patch is the first stop to making it easier for distros to ship
their own cross files (and for users to put their's somewhere safe so
they don't get `git clean`ed. It allows the cross files (on Linux and
*BSD) to be stored in home and system paths (~/.config/meson/cross,
/usr/share/meson/cross, and /usr/local/share/meson/cross), and to be
loaded by simply by specificying --cross-file.

With this patch meson will check the locations its always checked first,
(is cross file absolute, or is it relative to $PWD), then will check
~/.config/meson/cross, /usr/local/share/meson/cross,
/usr/share/meson/cross, (or $XDG_CONFIG_PATH and $XDG_DATA_DIRS) for the
files, raising an exception if it cannot find the specified cross file.

Fixes #2283

This exposes the already existing UserStringArrayOption class through
the meson_options.txt. The intention is to provide a way for projects to
take list/array type arguments and validate that all of the elements in
that array are valid without using complex looping constructrs.

Minor LLVM dependency cleanups for the next release

Followup for 1540e615f13503722f1067c693f3d394218cbd9e.

Add method to get values from config tool based dependency