Avoid using C++14 to build CMake if cstdio breaks

The GNU 4.8 standard library's cstdio header is not aware that C++14
honors C11's removal of "gets" from stdio.h and results in an error:

  /.../include/c++/4.8/cstdio:120:11: error: no member named 'gets' in the global namespace

Detect this problematic case and default to using C++11 instead of
C++14 for building CMake itself.

CMakeLists.txt

@@ -41,7 +41,12 @@ if(NOT DEFINED CMAKE_C_STANDARD AND NOT CMake_NO_C_STANDARD)
   set(CMAKE_C_STANDARD 11)
 endif()
 if(NOT DEFINED CMAKE_CXX_STANDARD AND NOT CMake_NO_CXX_STANDARD)
-  set(CMAKE_CXX_STANDARD 14)
+  include(${CMake_SOURCE_DIR}/Source/Checks/cm_cxx14_cstdio.cmake)
+  if(NOT CMake_CXX14_CSTDIO_BROKEN)
+    set(CMAKE_CXX_STANDARD 14)
+  else()
+    set(CMAKE_CXX_STANDARD 11)
+  endif()
 endif()
 
 # option to set the internal encoding of CMake to UTF-8

Source/Checks/cm_cxx14_cstdio.cmake

@@ -0,0 +1,33 @@
+set(CMake_CXX14_CSTDIO_BROKEN 0)
+if(CMAKE_CXX_COMPILER_ID MATCHES "GNU|Clang" AND CMAKE_CXX14_STANDARD_COMPILE_OPTION)
+  if(NOT DEFINED CMake_CXX14_CSTDIO_WORKS)
+    message(STATUS "Checking if compiler supports C++14 cstdio")
+    try_compile(CMake_CXX14_CSTDIO_WORKS
+      ${CMAKE_CURRENT_BINARY_DIR}
+      ${CMAKE_CURRENT_LIST_DIR}/cm_cxx14_cstdio.cpp
+      CMAKE_FLAGS -DCMAKE_CXX_STANDARD=14
+      OUTPUT_VARIABLE OUTPUT
+      )
+    if(CMake_CXX14_CSTDIO_WORKS AND "${OUTPUT}" MATCHES "error: no member named.*gets.*in the global namespace")
+      set_property(CACHE CMake_CXX14_CSTDIO_WORKS PROPERTY VALUE 0)
+    endif()
+    if(CMake_CXX14_CSTDIO_WORKS)
+      message(STATUS "Checking if compiler supports C++14 cstdio - yes")
+      file(APPEND ${CMAKE_BINARY_DIR}${CMAKE_FILES_DIRECTORY}/CMakeOutput.log
+        "Determining if compiler supports C++14 cstdio passed with the following output:\n"
+        "${OUTPUT}\n"
+        "\n"
+        )
+    else()
+      message(STATUS "Checking if compiler supports C++14 cstdio - no")
+      file(APPEND ${CMAKE_BINARY_DIR}${CMAKE_FILES_DIRECTORY}/CMakeError.log
+        "Determining if compiler supports C++14 cstdio failed with the following output:\n"
+        "${OUTPUT}\n"
+        "\n"
+        )
+    endif()
+  endif()
+  if(NOT CMake_CXX14_CSTDIO_WORKS)
+    set(CMake_CXX14_CSTDIO_BROKEN 1)
+  endif()
+endif()

Source/Checks/cm_cxx14_cstdio.cpp

@@ -0,0 +1,2 @@
+#include <cstdio>
+int main() { return 0; }