meson_options.txt: Support for reading options from meson.options

Support has been added for reading options from meson.options instead
of meson_options.txt[1]. These are equivalent, but not using t

meson_options.txt: Support for reading options from meson.options

Support has been added for reading options from meson.options instead
of meson_options.txt[1]. These are equivalent, but not using the .txt
extension for a build file has a few advantages, chief among them
many tools and text editors expect a file with the .txt extension to
be plain text files, not build scripts.

[1] https://mesonbuild.com/Release-notes-for-1-1-0.html#support-for-reading-options-from-mesonoptions

Signed-off-by: George Liu <liuxiwei@inspur.com>
Change-Id: Ida0b62337f249e7d96f58c2006c6ca4160825b6c

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