build: Fix meson test support

polkit's meson port was unable to execute unit test properly because
`mocklibc` support was missing.

This support has been added by using meson's wrap dependency system
feature, which is able to download `mocklibc` and the patch that
adds meson support to it. Thanks to this `polkit` is able to execute
unit tests properly. A new wrap file has also been merged[0].

This improvement has also hit an issue in the way meson handles every
project properties because they are inherited for every used
subproject[1]. In this case, `mocklibc` is not able to build properly
when some C standards and compiler options are used. To avoid this,
the C99 standaed flag and compiler options are used only in a project
scope, so `mocklibc` is able to build properly.

[0] https://github.com/mesonbuild/mocklibc/pull/1
[1] https://github.com/mesonbuild/meson/issues/1889
1 job for meson in 2 minutes and 26 seconds
latest
Status Job ID Name Coverage
  Test
passed #3880201
build_stable

00:02:26