* fsusage.m4 (gl_STATFS_TRUNCATES): Avoid unused variable.

* fsusage.m4 (gl_STATFS_TRUNCATES): Avoid unused variable.
* getdate.m4 (gl_C_COMPOUND_LITERALS): Likewise.
* jm-winsz1.m4 (gl_HEADER_TIOCGWINSZ_IN_TERMIOS_H): Likewise;
also add missing comma that caused broken test.
* link-follow.m4 (gl_AC_FUNC_LINK_FOLLOWS_SYMLINK): Include
stdlib.h, for `abort'.
* ls-mntd-fs.m4 (gl_LIST_MOUNTED_FILE_SYSTEMS): Avoid unused
variables.
* mkdir-slash.m4 (gl_FUNC_MKDIR_TRAILING_SLASH): Check for and
include unistd.h if present, for `rmdir'.
* physmem.m4 (gl_SYS__SYSTEM_CONFIGURATION): Avoid unused
variables.
* putenv.m4 (gl_FUNC_PUTENV): Rewrite using AC_RUN_IFELSE, and
in the process include standard headers for prototypes.
* readutmp.m4 (gl_READUTMP): Require AC_GNU_SOURCE, so utmpxname
gets declared on GNU/Linux.
* rmdir-errno.m4 (gl_FUNC_RMDIR_NOTEMPTY): Check for and include
unistd.h, for `rmdir'.
* time_r.m4 (gl_TIME_R): Avoid unused variables.