[3.9] bpo-41116: Fix setup.py test for macOS Tcl/Tk frameworks (GH-23649) (GH-23650)

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.
(cherry picked from commit 29afab6)

Co-authored-by: Ned Deily <[email protected]>

Author: miss-islington

Misc/NEWS.d/next/macOS/2020-12-04-23-09-11.bpo-41116.mSbXyV.rst

@@ -0,0 +1,3 @@
+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

@@ -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