Fix for a bug exposed by r45232:

/path/to/uninstalled/python setup.py build_ext

now failed with pyconfig.h not found.  Prior to r45232
the above command did not look for pyconfig.h, but the
bug is really in the look-up code: expecting to find it
in os.curdir is a rather fragile idea.

Lib/distutils/sysconfig.py

@@ -31,7 +31,7 @@ landmark = os.path.join(argv0_path, "Modules", "Setup")
 
 python_build = os.path.isfile(landmark)
 
-del argv0_path, landmark
+del landmark
 
 
 def get_python_version():
@@ -185,7 +185,7 @@ def customize_compiler(compiler):
 def get_config_h_filename():
     """Return full pathname of installed pyconfig.h file."""
     if python_build:
-        inc_dir = os.curdir
+        inc_dir = argv0_path
     else:
         inc_dir = get_python_inc(plat_specific=1)
     if get_python_version() < '2.2':