bpo-38360: macOS: support alternate form of -isysroot flag (GH-16480)

It is possible to use either '-isysroot /some/path' (with a space) or
'-isysroot/some/path' (no space in between). Support both forms in
places where special handling of -isysroot is done, rather than just
the first form.
Co-authored-by: Ned Deily <nad@python.org>

unixccompiler.py

@@ -288,7 +288,7 @@ class UnixCCompiler(CCompiler):
             # vs
             #   /usr/lib/libedit.dylib
             cflags = sysconfig.get_config_var('CFLAGS')
-            m = re.search(r'-isysroot\s+(\S+)', cflags)
+            m = re.search(r'-isysroot\s*(\S+)', cflags)
             if m is None:
                 sysroot = '/'
             else: