bug#4261: make gcc warn about c99 declarations

[Dan Nicolaescu]

There were several patch proposals recently that had c99 style
declarations in the middle of a block.
There's a gcc flag that warns about that.

The configure.in patch below enables the flag if gcc supports it,
following the model for -Wno-pointer-sign

Please apply if appropriate.



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 25 Aug 2009 23:11:58 -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