bug#6577: turn on -Wstrict-prototypes by default

[Dan Nicolaescu]

This patch turns on -Wstrict-prototypes to help transition to standard C.
[Archived here so that it does not get lost].

The error and message declarations in lisp.h need to be converted to
standard C (and the definition should use variable number of
arguments), otherwise this patch results in many warnings.

But if someone wants to check this in to speed up the process...

=== modified file 'configure.in'
--- configure.in  2010-07-02 12:55:51 +0000
+++ configure.in  2010-07-02 19:31:41 +0000
@@ -757,6 +757,20 @@ CFLAGS="$SAVE_CFLAGS"
 unset has_option
 unset SAVE_CFLAGS
 
+### Use -Wstrict-prototypes if the compiler supports it
+AC_MSG_CHECKING([whether gcc understands -Wstrict-prototypes])
+SAVE_CFLAGS="$CFLAGS"
+CFLAGS="$CFLAGS -Wstrict-prototypes"
+AC_TRY_COMPILE([], [], has_option=yes, has_option=no,)
+if test $has_option = yes; then
+   C_WARNINGS_SWITCH="-Wstrict-prototypes $C_WARNINGS_SWITCH"
+fi
+AC_MSG_RESULT($has_option)
+CFLAGS="$SAVE_CFLAGS"
+unset has_option
+unset SAVE_CFLAGS
+
+
 #### Some other nice autoconf tests.
 
 dnl checks for programs