[gtk-osx] Compute _target correctly for macOS 11 and later.

[John Ralls <jralls src gnome org>]

commit ef393942835ca3eea285c20b37a5a7c234b145ad
Author: John Ralls <jralls ceridwen us>
Date:   Sun Oct 31 14:49:37 2021 -0700

    Compute _target correctly for macOS 11 and later.

 jhbuildrc-gtk-osx | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)
---
diff --git a/jhbuildrc-gtk-osx b/jhbuildrc-gtk-osx
index 9d880b15..d74725c5 100644
--- a/jhbuildrc-gtk-osx
+++ b/jhbuildrc-gtk-osx
@@ -126,7 +126,10 @@ def osx_ver():
     exp = re.compile(r'(\d+\.\d+)\.\d+')
     vernum = exp.match(vstring)
     _osx_version = float(vernum.group(1)) - 4.0
-    _target = '10.%d' % int(_osx_version)
+    if _osx_version < 16:
+        _target = '10.%d' % int(_osx_version)
+    else:
+        _target = '%d' % (_osx_version - 5)
 
     _default_arch = _popen("uname -m")