(One supposes this is related to the closed bug 1044943.) Ubuntu 14.10, AMD FX4100 (quad-core), non-SSD, lots of RAM. Compiling the spidermonkey shell by itself I intermittently run into this error very early in the compilation (in release builds only): /home/lth/moz/mozilla-inbound/config/rules.mk:750: recipe for target 'host_jskwgen' failed When it occurs, simply restarting the compile fixes it. At least one other team member reports seeing this "all the time".
BTW, this is on mozilla-inbound, and it *never* happened to me until today, when I pulled new sources on coming back from the holiday break.
Haven't seen this in a very long time.