[gtk] (6 commits) ...Merge branch 'wip/lantw/improve-freebsd-build-and-fix-warnings' into 'master'

[Matthias Clasen <matthiasc src gnome org>]

Summary of changes:

  f922109... build: Make the default setting work on non-Linux Unix-like (*)
  ff1dd50... build: Use cc.links to check linker arguments (*)
  77637dc... Fix header guards (*)
  d3f40dc... imcontextxim: Don't assign a variable to itself (*)
  43a7f36... gtk-demo, testgtk: Don't pass an integer to fabs (*)
  9178f74... Merge branch 'wip/lantw/improve-freebsd-build-and-fix-warni

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