Detect Clang compiler version with its id

Decode decimal digits from

  __clang_major__
  __clang_minor__
  __clang_patchlevel__

to compute version number components.  See documentation at:

  http://clang.llvm.org/docs/LanguageExtensions.html#builtinmacros
  http://predef.sourceforge.net/precomp.html

Modules/CMakeCCompilerId.c.in

@@ -17,6 +17,9 @@
 
 #elif defined(__clang__)
 # define COMPILER_ID "Clang"
+# define COMPILER_VERSION_MAJOR DEC(__clang_major__)
+# define COMPILER_VERSION_MINOR DEC(__clang_minor__)
+# define COMPILER_VERSION_PATCH DEC(__clang_patchlevel__)
 
 #elif defined(__BORLANDC__)
 # define COMPILER_ID "Borland"

Modules/CMakeCXXCompilerId.cpp.in

@@ -19,6 +19,9 @@
 
 #elif defined(__clang__)
 # define COMPILER_ID "Clang"
+# define COMPILER_VERSION_MAJOR DEC(__clang_major__)
+# define COMPILER_VERSION_MINOR DEC(__clang_minor__)
+# define COMPILER_VERSION_PATCH DEC(__clang_patchlevel__)
 
 #elif defined(__BORLANDC__)
 # define COMPILER_ID "Borland"