-Wdeclaration-after-statement is gcc 4-only.

Index: configure.ac
--- configure.ac.orig
+++ configure.ac
@@ -17,7 +17,7 @@ PKG_CHECK_MODULES([X], [x11])
 dnl Add some useful warnings if we have gcc.
 dnl changequote(,)dnl
 AS_IF([test "x$ac_cv_c_compiler_gnu" = xyes], [
-  CFLAGS="$CFLAGS -fno-strict-aliasing -Wall -Wdeclaration-after-statement -Wno-unused"
+  CFLAGS="$CFLAGS -fno-strict-aliasing -Wall -Wno-unused"
 ])
 dnl changequote([,])dnl