api_internal includes and FreeBSD 12

[jdeblese]

api_internal fails to compile on FreeBSD 12.2 due to issues with included headers: alloca() is provided by stdlib.h instead of alloca.h (which doesn't exist); and the endian functions are already defined in sys/endian.h (included by stdint.h). A simple fix would be to check for the __FreeBSD__ macro and not include alloca.h or define the endianness functions when set. I can submit a PR, if that's an acceptable solution.