Throw away MS-DOS crap: we don't need getpid.
* configure.in: We don't need strndup. It was even causing
problems: because Flex includes the headers *before* us,
_GNU_SOURCE is not defined by config.h, and therefore strndup was
not visible.
* lib/xstrndup.c: New.
* src/scan-skel.l: Use it.
Be sure to initialize yylval.muscle member when scanning a MUSCLE.
* src/parse-skel.y: Use %directives instead of #defines.
