build: Attempt at delaying the build of ObjC files

Let's try keeping the dependency straight, and only copy the .m files
into .c files when we actually need them, instead of unconditionally.
6 jobs for objc-build in 8 minutes and 18 seconds (queued for 3 seconds)
latest
Status Job ID Name Coverage
  Build
passed #51787
fedora-meson-android_ndk_r16_api21_arm64

00:06:05

passed #51788
fedora-meson-mingw64

00:07:04

passed #51786
fedora-meson-x86_64

00:03:20

passed #51789
win32
msys2-mingw32

00:05:51

passed #51790
win32
vs2017-x64

00:05:48

 
  Coverage
passed #51791
coverage

00:01:11

77.0%