$(subst .c,.o,...) replaces '.c' with '.o' everywhere in pathnames. For
example, renaming the "Aegisub" folder to "Aegisub.cool" will make the
build system generate "Aegisub.oool/.../.o" objects.
https://www.gnu.org/software/make/manual/make.html#Text-Functions
In preparation for switching to LuaJIT, which doesn't support PUC Lua's
thing of using C++ exceptions for lua_error.
Requires replacing all uses of lua_error (and things calling lua_error)
with custom versions that throw an exception instead and adding an
exception -> lua error wrapper at all C++ -> Lua boundaries.
therealfun