Misc/NEWS.d/next/Library/2019-03-08-13-32-21.bpo-36235._M72wU.rst

Fix ``CFLAGS`` in ``customize_compiler()`` of ``distutils.sysconfig``: when
the ``CFLAGS`` environment variable is defined, don't override ``CFLAGS``
variable with the ``OPT`` variable anymore. Initial patch written by David
Malcolm.