[libglnx] (7 commits) ...Merge branch 'wip/smcv/subproject' into 'master'

[Phaedrus Leeds <mwleeds src gnome org>]

Summary of changes:

  b76cf45... meson: Automatically link libglnx-dependent objects to gio- (*)
  eeea247... meson: Always build with hidden symbol visibility (*)
  c500c36... tests: Build libglnx-testlib as a separate static library (*)
  96ba9f4... tests: Optionally skip building and running the actual test (*)
  4d5b1fc... tests: Verify libglnx can work as a subproject (*)
  238f7bf... fdio: Skip glnx_try_fallocate() if _GNU_SOURCE is not defin (*)
  72e532d... Merge branch 'wip/smcv/subproject' into 'master'

(*) This commit already existed in another branch; no separate mail sent