Log Message:
boehm-gc: update to 7.6.4.

== [7.6.4] 2018-01-26 ==

* Add note of set_free_space_divisor, set_warn_proc ABI change after gc-7.1
* Change compiler invocation example in gc.man to use dynamic libgc
* Delete dont_ar_* build intermediate files on make clean (Makefile.direct)
* Do not declare dl_iterate_phdr as weak for DragonFly
* Fix 'cords' parallel build in Makefile.direct
* Fix 'undeclared identifier USRSTACK' compiler error on OpenBSD-6.2
* Fix error code in abort message if sem_wait failed in start_world (NetBSD)
* Fix GC allocation mutex in child after a fork
* Fix global operator delete definition for C++14 in gc_cpp
* Fix last_reclaimed..gc_no interval comparison to threshold in unmap_old
* Fix libgc version which was changed in linkage breaking way
* Fix missing EOLn output in threadlibs tool
* Fix threadlibs tool to output '-lpthread' for DragonFly
* Prevent DATASTART redefinition for NaCl
* Remove obsolete advice about linking with _DYNAMIC=0 (Linux)