Allow CPPFLAGS to be set on the make command line

While looking into PR build/8751 (which seems to be fixed), I noticed
that it's not possible to change CPPFLAGS for gdb on the "make"
command line.  It's reasonable to want to do this sometimes, and I
think this patch should suffice.

gdb/ChangeLog
2018-08-07  Tom Tromey  <tom@tromey.com>

	* Makefile.in (CPPFLAGS): New variable.
	(INTERNAL_CPPFLAGS): Use it.