A platform makefile should define the following variables:

PLATFORM_DEBUG_FLAGS
	The flags to pass to the C# compiler to generate appropriate
	debug information

PLATFORM_MCS_FLAGS
	Flags that are always passed to the C# compiler

PLATFORM_RUNTIME
	The system runtime for executing CLR programs. Blank on Win32,
	probably $(RUNTIME) (the Mono runtime) on everything else.

PLATFORM_CORLIB
	The name of the system's corlib. 'mscorlib.dll'

PLATFORM_MAKE_CORLIB_CMP
	Define it to something nonempty if corlib_cmp.dll should be built
	on this platform. Only should be done if the platform provides its
	own non-Mono runtime that we can compare corlibs with.

PLATFORM_TWEAK_CORLIB_SOURCES
	A command to futz around with the sources to mscorlib.dll. See linux.make
	and win32.make for examples. Should be 'cat' unless you know what you're
	doing.

PLATFORM_CHANGE_SEPARATOR_CMD
	A command to change filename separators given on stdin. Leave undefined
	if Unixy filenames will work. See win32.make.

BOOTSTRAP_MCS
	The C# compiler used to compile mcs/mcs.exe, and possibly other
	assemblies if the user is doing a standalone build (that is, not
	a bootstrap).

hidden_prefix, hidden_suffix
	Prefix and suffix for hidden/temporary files. Unused right now, I think.

platform-check:
	A target to make sure that we can build on this platform.