(EMPTH_LWP, EMPTH_POSIX, EMPTH_W32): Define in config.h instead of

(EMPTH_LWP, EMPTH_POSIX, EMPTH_W32): Define in config.h instead of
compiler command line.  Don't bother to conditionalize code in .c
files that are only compiled when their thread package is used.

(_EMPTH_LWP, EMPTH_LWP, _EMPTH_POSIX, EMPTH_POSIX, _EMPTH_WIN32)
(EMPTH_W32): Identifiers beginning with an underscore and an uppercase
letter are reserved for any use.  Rename.