Add FreeBSD platform makefile

Add platform-freebsd.mk makefile to allow building on FreeBSD.

OpenH264 builds and runs successfully on FreeBSD 9.2-RELEASE amd64, with both
the gcc and clang compilers.

Note that building i386 binaries with the native toolchain using the -m32 flag
on amd64 FreeBSD is unsupported and will not work, so ENABLE64BIT should always
be set to Yes when compiling on a FreeBSD amd64 system.