Clean up scanners a bit.

* src/flex-scanner.h (FLEX_NO_OBSTACK): New macro that blocks obstack
definitions so gcc won't warn when obstack_for_string is unused.
* src/scan-code.l: config.h and system.h are already #include'd by
scan-code-c.c, so get rid of them here.
* src/scan-gram.l: Likewise.
* src/scan-skel.l: Likewise, and use flex-scanner.h without obstack
definitions rather than duplicating the rest of it.
* src/scan-gram-c.c, scan-skel-c.c: #include "system.h".

src/scan-code.l

@@ -24,17 +24,15 @@
 %option prefix="code_" outfile="lex.yy.c"
 
 %{
-#include <config.h>
-#include "system.h"
-
 /* Work around a bug in flex 2.5.31.  See Debian bug 333231
    <http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=333231>.  */
 #undef code_wrap
 #define code_wrap() 1
 
 #define FLEX_PREFIX(Id) code_ ## Id
-#include "complain.h"
 #include "flex-scanner.h"
+
+#include "complain.h"
 #include "reader.h"
 #include "getargs.h"
 #include <assert.h>