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.

1 files changed, 4 insertions, 4 deletions

diff --git a/src/su.c b/src/su.c
index f6b61f773..9f99b7871 100644
--- a/src/su.c
+++ b/src/su.c
@@ -118,10 +118,10 @@
 /* The user to become if none is specified.  */
 #define DEFAULT_USER "root"
 
-char *crypt ();
-char *getusershell ();
-void endusershell ();
-void setusershell ();
+char *crypt (char const *key, char const *salt);
+char *getusershell (void);
+void endusershell (void);
+void setusershell (void);
 
 extern char **environ;