is_windows returns TRUE if the OS on the
current platform is Microsoft windows-based.
is_unix returns TRUE if the OS is Unix
based (pretty much anything that isn't Windows, including
OS X). is_mac and is_linux return
TRUE if the OS is Linux or Apple OS X
respectively. is_64_bit_os returns TRUE
when the operating system is 64-bit. The assert_*
functions return nothing but throw an error if the
corresponding is_* functions return FALSE.