The custom C++ preprocessor thingy has been choking for a while on
clang; recently gcc's jointed the party. Work around this by building
it with -g instead of -O2. (For me at least with gcc, while -O2
produces a tool that gives spurious syntax errors, plain -O gives one
that loops forever instead, which isn't an improvement.)
Maybe someday someone will bother to find out where the undefined
behavior is.
Also build the actual game with -Wno-return-type to silence complaints
about dead functions not declared as dead and similar, and while here
adjust things so it still installs correctly regardless of umask.
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