Clean up scanners a bit.

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".