Add CheckLanguage module

Define a "check_language(<lang>)" macro to test whether <lang> can be
enabled.  Cache the result in CMAKE_<lang>_COMPILER.  Add a test case
covering expected results.

Modules/CheckLanguage.cmake

@@ -0,0 +1,65 @@
+# - Check if a language can be enabled
+# Usage:
+#  check_language(<lang>)
+# where <lang> is a language that may be passed to enable_language()
+# such as "Fortran".  If CMAKE_<lang>_COMPILER is already defined the
+# check does nothing.  Otherwise it tries enabling the language in a
+# test project.  The result is cached in CMAKE_<lang>_COMPILER as the
+# compiler that was found, or NOTFOUND if the language cannot be enabled.
+#
+# Example:
+#  check_language(Fortran)
+#  if(CMAKE_Fortran_COMPILER)
+#    enable_language(Fortran)
+#  else()
+#    message(STATUS "No Fortran support")
+#  endif()
+
+#=============================================================================
+# Copyright 2009-2012 Kitware, Inc.
+#
+# Distributed under the OSI-approved BSD License (the "License");
+# see accompanying file Copyright.txt for details.
+#
+# This software is distributed WITHOUT ANY WARRANTY; without even the
+# implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
+# See the License for more information.
+#=============================================================================
+# (To distribute this file outside of CMake, substitute the full
+#  License text for the above reference.)
+
+macro(check_language lang)
+  if(NOT DEFINED CMAKE_${lang}_COMPILER)
+    set(_desc "Looking for a ${lang} compiler")
+    message(STATUS ${_desc})
+    file(REMOVE_RECURSE ${CMAKE_CURRENT_BINARY_DIR}/CMakeFiles/Check${lang})
+    file(WRITE "${CMAKE_CURRENT_BINARY_DIR}/CMakeFiles/Check${lang}/CMakeLists.txt"
+      "cmake_minimum_required(VERSION 2.8)
+project(Check${lang} ${lang})
+file(WRITE \"\${CMAKE_CURRENT_BINARY_DIR}/result.cmake\"
+  \"set(CMAKE_${lang}_COMPILER \\\"\${CMAKE_${lang}_COMPILER}\\\")\\n\"
+  )
+")
+    execute_process(
+      WORKING_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR}/CMakeFiles/Check${lang}
+      COMMAND ${CMAKE_COMMAND} . -G ${CMAKE_GENERATOR}
+      OUTPUT_VARIABLE output
+      ERROR_VARIABLE output
+      RESULT_VARIABLE result
+      )
+    include(${CMAKE_CURRENT_BINARY_DIR}/CMakeFiles/Check${lang}/result.cmake OPTIONAL)
+    if(CMAKE_${lang}_COMPILER AND "${result}" STREQUAL "0")
+      file(APPEND ${CMAKE_BINARY_DIR}${CMAKE_FILES_DIRECTORY}/CMakeOutput.log
+        "${_desc} passed with the following output:\n"
+        "${output}\n")
+    else()
+      set(CMAKE_${lang}_COMPILER NOTFOUND)
+      file(APPEND ${CMAKE_BINARY_DIR}${CMAKE_FILES_DIRECTORY}/CMakeError.log
+        "${_desc} failed with the following output:\n"
+        "${output}\n")
+    endif()
+    message(STATUS "${_desc} - ${CMAKE_${lang}_COMPILER}")
+    set(CMAKE_${lang}_COMPILER "${CMAKE_${lang}_COMPILER}" CACHE FILEPATH "${lang} compiler")
+    mark_as_advanced(CMAKE_${lang}_COMPILER)
+  endif()
+endmacro()

Tests/CMakeOnly/CMakeLists.txt

@@ -17,6 +17,8 @@ add_CMakeOnly_test(CheckCXXSymbolExists)
 
 add_CMakeOnly_test(CheckCXXCompilerFlag)
 
+add_CMakeOnly_test(CheckLanguage)
+
 add_CMakeOnly_test(AllFindModules)
 
 add_CMakeOnly_test(TargetScope)

Tests/CMakeOnly/CheckLanguage/CMakeLists.txt

@@ -0,0 +1,22 @@
+cmake_minimum_required (VERSION 2.8)
+project(CheckLanguage NONE)
+include(CheckLanguage)
+
+set(langs )
+set(expect_C 1)
+set(expect_CXX 1)
+unset(expect_Fortran)
+set(expect_NoSuchLanguage 0)
+foreach(lang C CXX Fortran NoSuchLanguage)
+  check_language(${lang})
+  if(NOT DEFINED CMAKE_${lang}_COMPILER)
+    message(FATAL_ERROR "check_language(${lang}) did not set result")
+  endif()
+  if(DEFINED expect_${lang})
+    if(expect_${lang} AND NOT CMAKE_${lang}_COMPILER)
+      message(FATAL_ERROR "check_language(${lang}) should not fail!")
+    elseif(NOT expect_${lang} AND CMAKE_${lang}_COMPILER)
+      message(FATAL_ERROR "check_language(${lang}) should not succeed!")
+    endif()
+  endif()
+endforeach()