Ordering problem with --with-includes

I find that if one specifies, say,
configure --with-includes=/usr/local/include

one gets compiler commands like
cc -Ae -g +z -I/usr/local/include -I../../../src/include -I../../../src/interfaces/libpq  -c -o pgtcl.o pgtcl.c

because the -I commands are added to CPPFLAGS which appears before any
-I commands the makefiles themselves add.  This strikes me as uncool.
For example, it will be impossible to compile Postgres if there are
headers from an old version lurking in /usr/local/include, because those
will be read instead of the ones from our source tree.  How hard would
it be to make the --with-includes -I directives appear after our own?

The same problem arises for --with-libs, btw.
        regards, tom lane