dependencies: allow get_variable to expose files from subprojects

There are somewhat common, reasonable and legitimate use cases for a
dependency to provide data files installed to /usr which are used as
command inputs. When getting a dependency from a subproject, however,
the attempt to directly construct an input file from a subproject
results in a sandbox violation. This means not all dependencies can be
wrapped as a subproject.

One example is wayland-protocols XML files which get scanned and used to
produce C source files.

Teach Meson to recognize when a string path is the result of fetching a
dep.get_variable(), and special case this to be exempt from subproject
violations.

A requirement of this is that the file must be installed by
install_data() or install_subdir() because otherwise it is not actually
representative of what a pkg-config dependency would provide.

0 files changed, 0 insertions, 0 deletions