build: Use .m files directly

If we're on macOS we can count on an Objective C compiler to exist.
6 jobs for objc-check in 21 minutes and 33 seconds (queued for 3 seconds)
latest
Status Job ID Name Coverage
  Build
passed #51793
fedora-meson-android_ndk_r16_api21_arm64

00:06:01

passed #51794
fedora-meson-mingw64

00:15:45

passed #51792
fedora-meson-x86_64

00:03:22

passed #51795
win32
msys2-mingw32

00:05:38

passed #51796
win32
vs2017-x64

00:05:59

 
  Coverage
passed #51797
coverage

00:05:43

77.0%