Adds basic clang 6 support. Saddly I couldn't get spidermonkey 38 to compile
with clang 6, so instead I compile spidermonkey with gcc.
The latest spidermonkey from the gecko-dev respitory does compile however,
but updating the game to use it will take an painfull amount of time.
Diff 3856 shows my attempts at upgrading to the latest, however I got stuck on
replacing the [gs]etProperty functions from the JSClasses to proxies because
A) I have no clue what 0ad's code does B) I have no clue what the
proxies do and C) After spending a couple weeks I'm sick of updating code.