Add macosx CI

Fixes #265.
7 jobs for macosx-ci in 22 minutes and 6 seconds (queued for 6 seconds)
Status Job ID Name Coverage
  Build
passed #37930
fedora-meson-android_ndk_r16_api21_arm64

00:02:16

passed #37931
fedora-meson-mingw64

00:02:42

passed #37929
fedora-meson-x86_64

00:09:35

passed #37932
macosx-10.13
macosx-10.13-meson-x86_64

00:07:04

passed #37933
win32
msys2-mingw32

00:06:53

passed #37934
win32
vs2017-x64

00:06:21

 
  Coverage
passed #37935
coverage

00:01:13

76.7%