Changed from eager parsing of Makefile (at import time) to lazy: only do

Changed from eager parsing of Makefile (at import time) to lazy: only do
all that work when someone asks for a "configuration variable" from the
Makefile.  Details:
  - added 'get_config_vars()': responsible for calling one of the
    '_init_*()' functions to figure things out for this platform,
    and to provide an interface to the resulting dictionary
  - added 'get_config_var()' as a simple interface to the dictionary
    loaded by 'get_config_vars()'
  - changed the '_init_*()' functions so they load the global dictionary
    '_config_vars', rather than spewing their findings all over
    the module namespace
  - don't delete the '_init_*()' functions when done importing
  - adjusted 'customize_compiler()' to the new regime