Document how to build a 32-bit Mercury compiler on 64-bit Linux.

README.Linux:
	As above.
Avoid using llvm-gcc on Mac OS X.

The llvm-gcc on recent versions of Mac OS X is broken and cannot compile
Mercury.  This is inconvenient since the executable named `gcc' on these
versions of Mac OS X is llvm-gcc, not real gcc.

configure.ac:
     On Mac OS X, if we appear to be using llcm-gcc emit an error message.
Document how to build a 32-bit Mercury compiler on 64-bit Linux.

README.Linux:
        As above.
Fix an error in README.Linux.

README.Linux:
	Fix my confusion with autoconf options; using --host
	when cross-compiling is correct.
Remove the .mdbcomp_modules files in 'realclean'

In slice/ and deep_profiler/, when 'realclean' was executed
it removed copies of the .m files from mdbcomp/ but did not remove the
.mdbcomp_modules file.  When 'make' was executed again the .m files would
not be copied back into either slice/ or deep_profiler/

This patch fixes this problem by removing .mdbcomp_modules when 'realclean'
is executed.

deep_profiler/Mmakefile:
slice/Mmakefile:
    As above.
Add the .mdbcomp_modules files to .gitignore

deep_profiler/.gitignore:
slice/.gitignore:
    As above.