Add executable extension, needed to get the program name right on Win32

    Provide include_dirs argument to all calls to ._preprocess and ._compile
    Fix typo: pattern.search(pattern) should be pattern.search(line)

(This command seems to be essentially untested; should fix that...)

an 'other_libraries()' parameter.

Added 'check_lib()', which provides a subset of the functionality of
  'check_func()' with a simpler interface and implementation.

'try_cpp()', 'search_cpp()', and 'check_header()'.  This is enough that
the base config is actually useful for implementing a real config
command, specifically one for mxDateTime.

one doesn't *do* anything by default; it's just there as a conduit for data
(eg. include dirs, libraries) from the user to the "build" commands.
However, it provides a couple of Autoconf-ish methods ('try_compile()',
'try_link()', 'try_run()') that derived, per-distribution "config" commands
can use to poke around the target system and see what's available.

Initial experimenst with mxDateTime indicate that higher-level methods are
necessary: analogs of Autoconf's AC_CHECK_HEADER, AC_CHECK_LIB will be
needed too (and that's just to probe the C/C++ system: how to probe the
Python system is wide open, and someday we'll have to worry about probing a
Java system too).