In gdb/gdbserver/Makefile.in, IPAGENT_CFLAGS is defined using
an expression which references $(CPPFLAGS). But CPPFLAGS isn't
actually defined.
This patch first adds a CPPFLAGS definition, so as to inherit
the value passed at configure time (if any). And it then makes it
part of INTERNAL_CFLAGS_BASE, instead. There is no reason that
CPPFLAGS be useful for a certain class of source files, and not
the rest. This is also consistent with what's done in GDB.