Fixed Mac OS X 10.7 as a build platform (for GCC 4).

* Use gcc and g++ rather than cc and c++, as the latter now point to
  clang with recent Xcode versions and compilation of the host tools
  fail for various reasons with it.
* Replace the case-sensitive filesystem check with a more basic one,
  as diskutil no longer supports the behaviour of getting info for the
  volume that any path is on.
* Updated ReadMe with a correct list of prerequisites for OS X.
* GCC 2 builds are still broken due to a strange error that only
  occurs with a GCC 2 built on OS X 10.7

build/jam/BuildSetup

@@ -609,6 +609,13 @@ if $(HOST_PLATFORM) = linux || $(HOST_PLATFORM) = freebsd
 	}
 }
 
+# On recent versions of OS X cc and c++ point to clang, which fails to
+# compile some of the host tools. As a workaround, force use of GCC.
+if $(HOST_PLATFORM) = darwin {
+	HOST_CC = gcc ;
+	HOST_C++ = g++ ;
+}
+
 if $(HOST_PLATFORM) = cygwin {
 	HOST_LINKFLAGS += -Xlinker --allow-multiple-definition -Xlinker
 		--enable-auto-import ;