Enable -Wpointer-sign by default.

Enable -Wpointer-sign by default.

This enables -Wpointer-sign by default.

I've checked that --enable-targets=all builds fine with the following
as --host, on x86_64 Fedora 17 --build:

x86_64 GNU/Linux
i386 GNU/Linux
i386 MinGW-w64
i386 msdos/djgpp

OK?

gdb/
2013-04-19  Pedro Alves  <palves@redhat.com>

	* configure.ac (build_warnings): Replace -Wno-pointer-sign with
	-Wpointer-sign.
	* configure: Regenerate.

gdb/doc
2013-04-19  Pedro Alves  <palves@redhat.com>

	* gdbint.texinfo (Misc Guidelines) <Compiler Warnings>: Replace
	-Wno-pointer-sign text with text on -Wpointer-sign.