Fix problems with the Mercury compiler in the C# grade.

Fix some problems that prevent the Mercury compiler building and running in the
C# grade.  "Running" here means that ./mercury_compile --help works; I haven't
tried anything more ambitious (yet).

mdbcomp/shared_utilities.m:
	Add a Mercury definition of unlimit_stack/2 for use by the non-C
	backends.

runtime/mercury.h:
	Do not cause a preprocessor error if we are compiling with high-level
	code and without the Boehm GC if we are using clang.
	We need this on OS X because we include this header when building the
	programs in the util directory and on OS X we will probably be using
	clang rather than GCC.  (Clang and GCC both support the required
	extensions to C here, so this won't be a problem.)