setup script) to be a list of Extension instances, rather than a list of of
(ext_name, build_info) tuples.  This is mostly a simplification, but
'check_extension_list()' got a lot more complicated because of the need to
convert the old-style tuples to Extension instances.

Temporarily dropped support for defining/undefining macros in the
'extensions' list -- I want to change the interface, but haven't yet made
the required changes in CCompiler and friends to support this nicely.

Also neatened up the code that merges 'extra_compile_flags' and the CFLAGS
environment variable.