Support using clang as a C compiler with Mercury -- currently only

Branches: main, 11.07

Support using clang as a C compiler with Mercury -- currently only
tested on Mac OS X.

Clean up the handling of C compilers in the configure script.

configure.in:
	Improve this script's ability to distinguish between different
	types of C compiler.  In particular, ensure that a clear
	distinction is maintained between clang and gcc.

	Fix an old XXX: don't rely on pattern matching on the value
	of $CC to determine the C compiler type.  This is particularly
	error prone now since one compiler is named "clang" and another
	"cl".

	Set optimization and warning flags for clang.
	(XXX this is currently very conservative; we need to go through
	them and work out what good defaults might be.)

	Don't link against libgcc if we are using clang.
	(XXX I don't think this is really necessary for gcc either in
	the great majority of cases.)

	Mark spots that still need updating with "XXX CLANG"; I will
	look into these separately.

aclocal.m4:
	Add a new macro that determines the C compiler type based
	on what builtin macros are defined -- unlike AC_PROG_CC
	this macro correctly distinguishes between gcc and clang.

	Add two new macros that determine version information for
	clang and for Visual C.

scripts/mgnuc.in:
scripts/ml.in:
	Handle clang

README.MacOS:
	Mention that clang can be used with Mercury on Mac OS X.

README.MacOS

@@ -7,7 +7,7 @@ Mercury on Mac OS X 10.5 and 10.6
 
 Mercury should build and install "out-of-the-box" on Mac OS X 10.5 or 10.6
 using Apple's gcc version 4.2.  This version of gcc is included with the
-Developer Tools.
+Developer Tools.  On Mac OS X 10.6 you may also use clang.
 
 The 'asm_fast*' and 'reg*' grades are not currently available on Mac OS X 10.5
 or 10.6.  The only low-level C grades available are the 'none*' grades.  The
@@ -23,8 +23,8 @@ for compiling applications.
 By default, 64-bit versions of the executables and libraries in the Mercury
 system will be installed on x86-64 machines running Mac OS X 10.6.  To build a
 32-bit installation on such a machine, you need to arrange to have the option
-"-m32" passed to gcc.  This can be done by invoking Mercury's configure script
-with the option:
+"-m32" passed to gcc or clang.  This can be done by invoking Mercury's
+configure script with the option:
 
     --with-cc="gcc -m32"
 
@@ -47,8 +47,7 @@ Mmake.params at the top-level of the source tree:
 
     MCFLAGS = --cross-compiling
 
-Mercury can currently only be compiled by gcc on Mac OS X.  It cannot currently
-be compiled with llvm-gcc or clang.
+Mercury cannot currently be compiled with llvm-gcc on Mac OS X.
 
 If, after installing Mercury, you encounter errors about missing .mih files,
 and you have fink installed, then try removing the fink components from your