tsystem.h: Test only POSIX for availability of string.h and time.h.

tsystem.h: Test only POSIX for availability of string.h and time.h.

	* tsystem.h: Test only POSIX for availability of string.h
	and time.h.
	* config.gcc (all systems): If USG appeared in xm_defines,
	change it to POSIX.  If both USG and POSIX appeared, just
	delete USG. Remove unnecessary quotes around xm_defines
	settings.
	(m68k-crds-unos*): Don't #define unos - tested nowhere.
	(m68k-apple-aux*): Don't #define AUX - tested nowhere.
	(alpha*-*-linux*ecoff*, powerpc-*-linux*libc1,
	powerpc-*-linux*): No need to set xm_defines - handled by
	global settings for *-*-linux*.

From-SVN: r41714