build: add configure-time --enable-gcc-warnings option; avoid warnings

build: add configure-time --enable-gcc-warnings option; avoid warnings

* bootstrap.conf (gnulib_modules): Add "warnings" module.
* configure.ac: Add --enable-gcc-warnings, derived from code in bison.
* src/Makefile.am (AM_CFLAGS): Set to $(WARN_CFLAGS) # $(WERROR_CFLAGS)
* lib/Makefile.am (AM_CFLAGS): Change spelling to $(WARN_CFLAGS)
Don't use $(WERROR_CFLAGS), yet.
* src/system.h (usage): Declare.
* src/base64.c (usage): Declare to be global, for consistency.
* src/timeout.c (usage): Likewise.
* src/truncate.c (usage): Likewise.
* src/getlimits.c (usage): Likewise.
* src/pinky.c (ttyname): Declare with prototype, rather than an
empty argument list.
* src/who.c (ttyname): Likewise.
* src/su.c (crypt, getusershell, setusershell, endusershell): Likewise.