Add support for AppleClang and iOS cross compilation

Add support for AppleClang and iOS cross compilation

Compiling for iOS on an intel mac failed due to not handling the locations of
gen_defines.asm and asm_offset.s when running under AppleClang. This solution
works for the toolchain file used during development but may not work in other
situations.

iOS does not support secure_getenv, which is fine, except that the cmake
code check_function_exists seems to think it is supported, causing compilation
to fail. Just hardcoding secure_getenv to not be supported is enough for
the unix shim logic to compile.

The compilation revelaed a few issues in the test code, like not using
[[maybe_unused]] and a signed/unsigned comparison.