Fixed ugly workaround for compiler detection

Corrected ugly workaround for compiler detection in code with
correct definition in llpreprocessor.h and updated various #if
to reflect this.

indra/llcommon/llpreprocessor.h

@@ -78,7 +78,18 @@
 
 
 // Figure out differences between compilers
-#if defined(__GNUC__)
+#if defined(__clang__) && defined(__GNUC__)
+	#define CLANG_VERSION (__clang_major__ * 10000 \
+						+ __clang_minor__ * 100 \
+						+ __clang_patchlevel__)
+	#ifndef LL_CLANG
+		#define LL_CLANG 1
+	#endif
+#elif defined (__ICC) && defined(__GNUC__)
+	#ifndef LL_ICC
+		#define LL_ICC 1
+	#endif
+#elif defined(__GNUC__)
 	#define GCC_VERSION (__GNUC__ * 10000 \
 						+ __GNUC_MINOR__ * 100 \
 						+ __GNUC_PATCHLEVEL__)