bug#4434: add -Wdeclaration-after-statement to CFLAGS if supported

[Dan Nicolaescu <dann@ics.uci.edu>]

Recently we've had a few instances of declarations being added in the
middle of a block.

gcc has a flag to warn about this.

Here's a patch to add this flag to CFLAGS (it mirrors what we already do
for  -Wno-pointer-sign).

OK to check in?



Index: configure.in
===================================================================
RCS file: /cvsroot/emacs/emacs/configure.in,v
retrieving revision 1.607
diff -u -3 -p -u -p -r1.607 configure.in
--- configure.in 23 Aug 2009 02:15:00 -0000     1.607
+++ configure.in 14 Sep 2009 18:46:36 -0000
@@ -757,6 +757,19 @@ CFLAGS="$SAVE_CFLAGS"
 unset has_option
 unset SAVE_CFLAGS
 
+### Use -Wdeclaration-after-statement if the compiler supports it
+AC_MSG_CHECKING([whether gcc understands
-Wdeclaration-after-statement])
+SAVE_CFLAGS="$CFLAGS"
+CFLAGS="$CFLAGS -Wdeclaration-after-statement"
+AC_TRY_COMPILE([], [], has_option=yes, has_option=no,)
+if test $has_option = yes; then
+   C_WARNINGS_SWITCH="-Wdeclaration-after-statement $C_WARNINGS_SWITCH"
+fi
+AC_MSG_RESULT($has_option)
+CFLAGS="$SAVE_CFLAGS"
+unset has_option
+unset SAVE_CFLAGS
+
 #### Some other nice autoconf tests.
 
 dnl checks for programs