Recognize Clang C and C++ compilers (see #10693)

Map to the platform and compiler information for GNU because the
compilers are command-line compatible for common operations.  Later we
can add Clang-specific features as necessary.  We honor the preferred
capitalization is "Clang", not the common mis-spelling "CLang".

Modules/CMakeCCompilerId.c.in

@@ -9,6 +9,9 @@
 #if defined(__INTEL_COMPILER) || defined(__ICC)
 # define COMPILER_ID "Intel"
 
+#elif defined(__clang__)
+# define COMPILER_ID "Clang"
+
 #elif defined(__BORLANDC__)
 # define COMPILER_ID "Borland"
 

Modules/CMakeCXXCompilerId.cpp.in

@@ -11,6 +11,9 @@
 #elif defined(__INTEL_COMPILER) || defined(__ICC)
 # define COMPILER_ID "Intel"
 
+#elif defined(__clang__)
+# define COMPILER_ID "Clang"
+
 #elif defined(__BORLANDC__)
 # define COMPILER_ID "Borland"
 

Modules/Compiler/Clang-C.cmake

@@ -0,0 +1 @@
+include(Compiler/GNU-C)

Modules/Compiler/Clang-CXX.cmake

@@ -0,0 +1 @@
+include(Compiler/GNU-CXX)

Modules/Platform/Linux-Clang-C.cmake

@@ -0,0 +1 @@
+INCLUDE(Platform/Linux-GNU-C)

Modules/Platform/Linux-Clang-CXX.cmake

@@ -0,0 +1 @@
+INCLUDE(Platform/Linux-GNU-CXX)