bpo-41116: Fix setup.py test for macOS Tcl/Tk frameworks (GH-23649)

If no explicit macOS SDK was specified, setup.py should check for
Tcl and TK frameworks in /Library/Frameworks; the previous commit
inadvertently broke that test.

setup.py

diff --git a/setup.py b/setup.py
index 0c9a425..90588e8 100644
--- a/setup.py
+++ b/setup.py
@@ -177,10 +177,11 @@ def macosx_sdk_root():
     m = re.search(r'-isysroot\s*(\S+)', cflags)
     if m is not None:
         MACOS_SDK_ROOT = m.group(1)
+        MACOS_SDK_SPECIFIED = MACOS_SDK_ROOT != '/'
     else:
         MACOS_SDK_ROOT = _osx_support._default_sysroot(
             sysconfig.get_config_var('CC'))
-    MACOS_SDK_SPECIFIED = MACOS_SDK_ROOT != '/'
+        MACOS_SDK_SPECIFIED = False
 
     return MACOS_SDK_ROOT