The cause for the failure seems to be a bug in the dreaded _parse_makefile(). When you give it a Makefile such as:

LDFLAGS=-foo
PY_LDFLAGS=-bar
LDSHARED=$(CC) -shared ${LDFLAGS} $(PY_LDFLAGS)

It outputs the following variables:

{'CC': '',
 'LDFLAGS': '-foo',
 'LDSHARED': '-shared ${LDFLAGS} -bar',
 'PY_LDFLAGS': '-bar'}

That is, ${LDFLAGS} isn't expanded.

However, since the Makefile now explicitly appends PY_LDFLAGS to LDSHARED, we could also stop doing so in ./configure.