[Bug web/82686] Debian sid powerpc64-unknown-linux-gnu 4.13.0-1-powerpc64 bootstrap breaks in stage3 with unexpected requirement for bdw-gc

["dclarke at blastwave dot org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82686

--- Comment #4 from Dennis Clarke <dclarke at blastwave dot org> ---
So this is not at all clear about how to continue. I did install the
new requirement or at least the "undocumented" dependency from the 
Debian pkg tree :

nix:~# apt-get install libgc-dev
Reading package lists... Done
Building dependency tree       
Reading state information... Done
The following additional packages will be installed:
  libgc1c2
The following NEW packages will be installed:
  libgc-dev libgc1c2
0 upgraded, 2 newly installed, 0 to remove and 14 not upgraded.
Need to get 569 kB of archives.
After this operation, 1,553 kB of additional disk space will be used.
Do you want to continue? [Y/n] 
.
.
.


However the bootstrap still fails in the stage3 phase : 

.
.
.
checking whether the target supports thread-local storage... yes
checking if the type of bitfields matters... yes
checking for bdw garbage collector... checking for system boehm-gc...
configure: error: system bdw-gc required but not found
gmake[1]: *** [Makefile:22695: configure-target-libobjc] Error 1
gmake[1]: Leaving directory
'/usr/local/build/gcc-7.2.0_linux_4.13.0-1-powerpc64.003'
gmake: *** [Makefile:941: all] Error 2
Command exited with non-zero status 2
nix_$ 

nix_$ cat stage_current 
stage3

So what exactly is needed in order to get past this?  Documented or otherwise?