Fix CPPFLAGS handling in gdbserver's build.

Fix CPPFLAGS handling in gdbserver's build.

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.

gdb/gdbserver/ChangeLog:

        * Makefile.in (CPPFLAGS): Define.
        (INTERNAL_CFLAGS_BASE): Add ${CPPFLAGS}.
        (IPAGENT_CFLAGS): Remove ${CPPFLAGS}.

Tested by rebuilding GDBserver with a dummy CPPFLAGS, and verifying
that the compilation command was altered as expected.