[m-rev.] for review: Upgrade Boehm GC to v7.6.2

[Julien Fischer]

Hi Peter,

On Mon, 8 Jan 2018, Peter Wang wrote:

> I've only tested this some on Linux (glibc, musl).

THe update presumably (properly) fixes the problems with musl?

I have no objections to updating; I will do some additional testing
on OS X and Windows.

Julien.