* bpo-29243: Fix Makefile with respect to --enable-optimizations

When using the Profile Guided Optimization (./configure --enable-optimizations)
Python is built not only during `make` but rebuilt again during `make test`,
`make install` and others. This patch fixes the issue.

Note that this fix produces no change at all in the Makefile if configure is
run witout --enable-optimizations.

* !squash