Since none of the compiles(xxx) calls are to particularly complex code, we can
just use `rsconf` directly to test for the presence of the symbols or headers as
needed.
Note that it seems at least some of the previous detection was not working
correctly; in particular HAVE_PIPE2 was evaluating to false on my WSL install
where pipe2(2) was available (caught because it revealed some compilation errors
in that conditional compilation path after porting).
I kept the cfg names and the tests themselves mostly as-is, though we might want
to change that to conform with the rust convention of lowercase cfg names and
decide whether we want to prefix all these with have_, fish_, or nothing at all.
Also the posix_spawn() test should probably check for the symbol `posix_spawn()`
rather than the header `spawn.h` since we don't use it via the header but rather
via the symbol (but in reality they're almost certainly going to give the same
result).
