CMakeDetermine(C|CXX)Compiler: Consider Clang compilers

Look for "clang" or "clang++" compiler executables so Clang will be used
when it is the only compiler available.  Prefer them last to avoid
changing compiler default preferences for existing scripts.

Modules/CMakeDetermineCCompiler.cmake

@@ -56,7 +56,7 @@ IF(NOT CMAKE_C_COMPILER)
   IF(CMAKE_C_COMPILER_INIT)
     SET(CMAKE_C_COMPILER_LIST ${CMAKE_C_COMPILER_INIT})
   ELSE(CMAKE_C_COMPILER_INIT)
-    SET(CMAKE_C_COMPILER_LIST ${_CMAKE_TOOLCHAIN_PREFIX}gcc ${_CMAKE_TOOLCHAIN_PREFIX}cc cl bcc xlc)
+    SET(CMAKE_C_COMPILER_LIST ${_CMAKE_TOOLCHAIN_PREFIX}gcc ${_CMAKE_TOOLCHAIN_PREFIX}cc cl bcc xlc clang)
   ENDIF(CMAKE_C_COMPILER_INIT)
 
   # Find the compiler.

Modules/CMakeDetermineCXXCompiler.cmake

@@ -55,7 +55,7 @@ IF(NOT CMAKE_CXX_COMPILER)
   IF(CMAKE_CXX_COMPILER_INIT)
     SET(CMAKE_CXX_COMPILER_LIST ${CMAKE_CXX_COMPILER_INIT})
   ELSE(CMAKE_CXX_COMPILER_INIT)
-    SET(CMAKE_CXX_COMPILER_LIST ${_CMAKE_TOOLCHAIN_PREFIX}c++ ${_CMAKE_TOOLCHAIN_PREFIX}g++ CC aCC cl bcc xlC)
+    SET(CMAKE_CXX_COMPILER_LIST ${_CMAKE_TOOLCHAIN_PREFIX}c++ ${_CMAKE_TOOLCHAIN_PREFIX}g++ CC aCC cl bcc xlC clang++)
   ENDIF(CMAKE_CXX_COMPILER_INIT)
 
   # Find the compiler.