Check(Function|Library|Symbol)Exists: make it work if only C++ is enabled

Help/release/dev/Threads-CXX.rst

+Threads-CXX
+------------
+
+* The :module:`CheckFunctionExists`, :module:`CheckLibraryExists`, and
+  :module:`CheckSymbolExists` modules learned to work in environments where
+  only CXX is enabled.

Modules/CheckFunctionExists.c

 #ifdef CHECK_FUNCTION_EXISTS
 
+#ifdef __cplusplus
+extern "C"
+#endif
 char CHECK_FUNCTION_EXISTS();
 #ifdef __CLASSIC_C__
 int main(){

Modules/CheckFunctionExists.cmake

@@ -57,14 +57,26 @@ macro(CHECK_FUNCTION_EXISTS FUNCTION VARIABLE)
     else()
       set(CHECK_FUNCTION_EXISTS_ADD_INCLUDES)
     endif()
+
+    if(CMAKE_C_COMPILER_LOADED)
+      set(_cfe_source ${CMAKE_ROOT}/Modules/CheckFunctionExists.c)
+    elseif(CMAKE_CXX_COMPILER_LOADED)
+      set(_cfe_source ${CMAKE_BINARY_DIR}${CMAKE_FILES_DIRECTORY}/CheckFunctionExists/CheckFunctionExists.cxx)
+      configure_file(${CMAKE_ROOT}/Modules/CheckFunctionExists.c "${_cfe_source}" COPYONLY)
+    else()
+      message(FATAL_ERROR "CHECK_FUNCTION_EXISTS needs either C or CXX language enabled")
+    endif()
+
     try_compile(${VARIABLE}
       ${CMAKE_BINARY_DIR}
-      ${CMAKE_ROOT}/Modules/CheckFunctionExists.c
+      ${_cfe_source}
       COMPILE_DEFINITIONS ${CMAKE_REQUIRED_DEFINITIONS}
       ${CHECK_FUNCTION_EXISTS_ADD_LIBRARIES}
       CMAKE_FLAGS -DCOMPILE_DEFINITIONS:STRING=${MACRO_CHECK_FUNCTION_DEFINITIONS}
       "${CHECK_FUNCTION_EXISTS_ADD_INCLUDES}"
       OUTPUT_VARIABLE OUTPUT)
+    unset(_cfe_source)
+
     if(${VARIABLE})
       set(${VARIABLE} 1 CACHE INTERNAL "Have function ${FUNCTION}")
       if(NOT CMAKE_REQUIRED_QUIET)

Modules/CheckLibraryExists.cmake

@@ -53,15 +53,26 @@ macro(CHECK_LIBRARY_EXISTS LIBRARY FUNCTION LOCATION VARIABLE)
       set(CHECK_LIBRARY_EXISTS_LIBRARIES
         ${CHECK_LIBRARY_EXISTS_LIBRARIES} ${CMAKE_REQUIRED_LIBRARIES})
     endif()
+
+    if(CMAKE_C_COMPILER_LOADED)
+      set(_cle_source ${CMAKE_ROOT}/Modules/CheckFunctionExists.c)
+    elseif(CMAKE_CXX_COMPILER_LOADED)
+      set(_cle_source ${CMAKE_BINARY_DIR}${CMAKE_FILES_DIRECTORY}/CheckLibraryExists/CheckFunctionExists.cxx)
+      configure_file(${CMAKE_ROOT}/Modules/CheckFunctionExists.c "${_cle_source}" COPYONLY)
+    else()
+      message(FATAL_ERROR "CHECK_FUNCTION_EXISTS needs either C or CXX language enabled")
+    endif()
+
     try_compile(${VARIABLE}
       ${CMAKE_BINARY_DIR}
-      ${CMAKE_ROOT}/Modules/CheckFunctionExists.c
+      ${_cle_source}
       COMPILE_DEFINITIONS ${CMAKE_REQUIRED_DEFINITIONS}
       LINK_LIBRARIES ${CHECK_LIBRARY_EXISTS_LIBRARIES}
       CMAKE_FLAGS
       -DCOMPILE_DEFINITIONS:STRING=${MACRO_CHECK_LIBRARY_EXISTS_DEFINITION}
       -DLINK_DIRECTORIES:STRING=${LOCATION}
       OUTPUT_VARIABLE OUTPUT)
+    unset(_cle_source)
 
     if(${VARIABLE})
       if(NOT CMAKE_REQUIRED_QUIET)

Modules/CheckSymbolExists.cmake

@@ -44,10 +44,14 @@
 # (To distribute this file outside of CMake, substitute the full
 #  License text for the above reference.)
 
-
-
 macro(CHECK_SYMBOL_EXISTS SYMBOL FILES VARIABLE)
-  _CHECK_SYMBOL_EXISTS("${CMAKE_BINARY_DIR}${CMAKE_FILES_DIRECTORY}/CMakeTmp/CheckSymbolExists.c" "${SYMBOL}" "${FILES}" "${VARIABLE}" )
+  if(CMAKE_C_COMPILER_LOADED)
+    _CHECK_SYMBOL_EXISTS("${CMAKE_BINARY_DIR}${CMAKE_FILES_DIRECTORY}/CMakeTmp/CheckSymbolExists.c" "${SYMBOL}" "${FILES}" "${VARIABLE}" )
+  elseif(CMAKE_CXX_COMPILER_LOADED)
+    _CHECK_SYMBOL_EXISTS("${CMAKE_BINARY_DIR}${CMAKE_FILES_DIRECTORY}/CMakeTmp/CheckSymbolExists.cxx" "${SYMBOL}" "${FILES}" "${VARIABLE}" )
+  else()
+    message(FATAL_ERROR "CHECK_SYMBOL_EXISTS needs either C or CXX language enabled")
+  endif()
 endmacro()
 
 macro(_CHECK_SYMBOL_EXISTS SOURCEFILE SYMBOL FILES VARIABLE)