meson_options.txt: add host-gi, gi-cross-wrapper, gi-ldd-wrapper, introspection-data and pkgconfig-sysroot-path options

With the first option, gobject-introspection tools (g-ir-doc-tool and g-ir-scanner)
that are already installed in the host system will be used for building the source tree.

With the second option, g-ir-scanner will be instructed to use an executable
wrapper to run binaries it's producing, and g-ir-compiler will be run
through the same wrapper (host system's g-ir-compiler cannot be used because
it's producing architecture-specific output).

With the third option, giscanner will be instructed to use a special ldd
command instead of system's ldd (which does not work when the binary to inspect
is compiled for a different architecture).

With the fourth option, it is possible to disable building of introspection data
(.gir and .typelib files), which may be difficult or impossible in cross-compilation
environments, because of lack of emulation (or native hardware) for the target architecture
on which the target binaries can be run.

With the fifth option, paths returned by pkg-config are prefixed with the sysroot
path (which is the destination path for cross-compiled items on the system where
cross-compilation happens).

These options are useful when cross-compiling for a different target architecture.

Signed-off-by: Alexander Kanavin <alex.kanavin@gmail.com>