meson: Do not warn about xdg-email on macOS

There is no such thing from Freedesktop for macOS. What we
use (at packaging phase) is an AppleScript by Alex Samorukov
at build/macos/patches/xdg-email.

So, makes no sense having that warning on macOS since such
custom script does not exist anywhere to be found at build-time.

meson.build

@@ -1161,7 +1161,7 @@ else
   if xdg_email.found()
     xdg_email_path = xdg_email.full_path()
     email_message = '@0@ (@1@)'.format(true, xdg_email_path)
-  elif not xdg_email.found() and not platform_windows
+  elif not xdg_email.found() and not platform_windows and not platform_osx
     mail_warning = 'Xdg-email not found, but required at runtime for email sending.'
     warning(mail_warning)
     warnings += mail_warning