build: Mismatch between gint64 and int64_t in OSX 64-bit

Check for compile warnings when assigning an int64_t* to a long*,
make gint64 a long long if they occur and assigning an int64_t* to
a long long* doesn't.

Modified by Philip Withnall <withnall@endlessm.com> to support Meson as
well as autotools.

GNOME/glib#972
8 jobs for 972-integers-on-os-x in 13 minutes and 33 seconds (queued for 14 seconds)
Status Job ID Name Coverage
  Build
passed #88067
cross-android_api21_arm64

00:01:37

passed #88068
cross-android_api28_arm64

00:01:46

passed #88069
cross-mingw64

00:02:11

passed #88066
fedora-x86_64

00:04:39

manual #88072
freebsd-11 allowed to fail manual
freebsd-11-x86_64
passed #88070
win32
msys2-mingw32

00:07:29

passed #88071
win32
vs2017-x64

00:05:45

 
  Coverage
passed #88073
coverage

00:01:39

73.3%