meson: modify options for enabling the bindings

meson: modify options for enabling the bindings

bindings are added as subdir by foreaching a array defined in meson.build at root.
then meson checks a option which has the same name of the binding.
this patch appends a new option for selecting bindings to build.

[howto]
*as-is
meson build.asis/ -Dmono=false -Dcxx=true
ninja -C build.asis/

*to-be
menson build.tobe/ -Dbindings=luajit,cxx
ninja -C build.tobe/

it is imposibble to use this wrongly because meson raise a error if arguments are not in a predefined list that described in meson_options.txt.
for more information, refer to https://mesonbuild.com/Build-options.html and also take a look at meson_options.txt please.
Differential Revision: https://phab.enlightenment.org/D7563

Change-Id: I5fa0af9ecf9b76a15a40f2822f6f34c5e4602c3b
Signed-off-by: Wonki Kim <wonki_.kim@samsung.com>