HEAD doesn't build. Totally stalled.

I'm getting this failure in a clean HEAD build. Any ideas? I'm totally stalled because I can't build GHC any more. I'm using Windows Subsystem for Linux (WSL). Help help! Thanks Simon /home/simonpj/code/HEAD-9/rts/dist/build/libHSrts_thr_p.a(RtsSymbols.thr_p_o): RtsSymbols.c:rtsSyms: error: undefined reference to '__stack_chk_guard' collect2: error: ld returned 1 exit status `cc' failed in phase `Linker'. (Exit code: 1) utils/iserv/ghc.mk:105: recipe for target 'utils/iserv/stage2_p/build/tmp/ghc-iserv-prof' failed make[1]: *** [utils/iserv/stage2_p/build/tmp/ghc-iserv-prof] Error 1 make[1]: *** Waiting for unfinished jobs....