* fsusage.m4 (gl_FILE_SYSTEM_USAGE): Use return, not exit.

* fsusage.m4 (gl_FILE_SYSTEM_USAGE): Use return, not exit.
Don't assume that exit status -1 is valid.
* nanosleep.m4 (gl_FUNC_NANOSLEEP): Likewise.
* putenv.m4 (gl_FUNC_PUTENV): Likewise.
* rmdir-errno.m4 (gl_FUNC_RMDIR_NOTEMPTY): Likewise.
* readdir.m4 (GL_FUNC_READDIR): Include <stdlib.h>.
* rename.m4 (vb_FUNC_RENAME): Likewise.
* unlink-busy.m4 (gl_FUNC_UNLINK_BUSY_TEXT): Use AC_RUN_IFELSE,
not AC_TRY_RUN.  Use return, not exit.  Don't assume that
functions can be used without declaring them, or that you can
exit with status -1.
* utimes-null.m4 (gl_FUNC_UTIMES_NULL): Likewise.