Features: Fix the default C dialect for Clang and GNU.

Clang 3.4 uses C99 by default, and Clang 3.6 uses C11 by default:

 http://thread.gmane.org/gmane.comp.compilers.clang.devel/39379

GNU 4.9 uses C90 by default, and GNU 5.0 uses C11 by default:

 https://gcc.gnu.org/gcc-5/changes.html

Test that the default compiler settings result in the expected dialect
macros being defined for both C and CXX.  Remove the unused main.c
file from the CompileFeatures unit test.

Modules/Compiler/Clang-C.cmake

@@ -17,7 +17,11 @@ if(NOT CMAKE_C_COMPILER_VERSION VERSION_LESS 3.4)
   set(CMAKE_C11_EXTENSION_COMPILE_OPTION "-std=gnu11")
 endif()
 
-set(CMAKE_C_STANDARD_DEFAULT 90)
+if(NOT CMAKE_C_COMPILER_VERSION VERSION_LESS 3.6)
+  set(CMAKE_C_STANDARD_DEFAULT 11)
+elseif(NOT CMAKE_C_COMPILER_VERSION VERSION_LESS 3.4)
+  set(CMAKE_C_STANDARD_DEFAULT 99)
+endif()
 
 macro(cmake_record_c_compile_features)
   macro(_get_clang_features std_version list)

Modules/Compiler/GNU-C.cmake

@@ -12,8 +12,11 @@ if (NOT CMAKE_C_COMPILER_VERSION VERSION_LESS 4.7)
   set(CMAKE_C11_EXTENSION_COMPILE_OPTION "-std=gnu11")
 endif()
 
-# This may change in a future GNU version.
-set(CMAKE_C_STANDARD_DEFAULT 90)
+if (NOT CMAKE_C_COMPILER_VERSION VERSION_LESS 5.0)
+  set(CMAKE_C_STANDARD_DEFAULT 11)
+else(NOT CMAKE_C_COMPILER_VERSION VERSION_LESS 4.7)
+  set(CMAKE_C_STANDARD_DEFAULT 90)
+endif()
 
 macro(cmake_record_c_compile_features)
   macro(_get_gcc_features std_version list)

Tests/CompileFeatures/CMakeLists.txt

@@ -77,7 +77,29 @@ foreach(lang CXX C)
   endif()
 endforeach()
 
+if (CMAKE_C_COMPILE_FEATURES)
+  string(FIND "${CMAKE_C_FLAGS}" "-std=" std_flag_idx)
+  if (std_flag_idx EQUAL -1)
+    add_executable(default_dialect_C default_dialect.c)
+    target_compile_definitions(default_dialect_C PRIVATE
+      DEFAULT_C11=$<EQUAL:${CMAKE_C_STANDARD_DEFAULT},11>
+      DEFAULT_C99=$<EQUAL:${CMAKE_C_STANDARD_DEFAULT},99>
+      DEFAULT_C90=$<EQUAL:${CMAKE_C_STANDARD_DEFAULT},90>
+    )
+  endif()
+endif()
+
 if (CMAKE_CXX_COMPILE_FEATURES)
+  string(FIND "${CMAKE_CXX_FLAGS}" "-std=" std_flag_idx)
+  if (std_flag_idx EQUAL -1)
+    add_executable(default_dialect default_dialect.cpp)
+    target_compile_definitions(default_dialect PRIVATE
+      DEFAULT_CXX14=$<EQUAL:${CMAKE_CXX_STANDARD_DEFAULT},14>
+      DEFAULT_CXX11=$<EQUAL:${CMAKE_CXX_STANDARD_DEFAULT},11>
+      DEFAULT_CXX98=$<EQUAL:${CMAKE_CXX_STANDARD_DEFAULT},98>
+    )
+  endif()
+
   add_executable(CompileFeatures main.cpp)
   set_property(TARGET CompileFeatures
     PROPERTY COMPILE_FEATURES "cxx_auto_type"

Tests/CompileFeatures/default_dialect.c

@@ -0,0 +1,22 @@
+
+#if DEFAULT_C11
+#  if __STDC_VERSION__ != 201112L
+#    error Unexpected value for __STDC_VERSION__.
+#  endif
+#elif DEFAULT_C99
+#  if __STDC_VERSION__ != 199901L
+#    error Unexpected value for __STDC_VERSION__.
+#  endif
+#else
+#  if !DEFAULT_C90
+#    error Buildsystem error
+#  endif
+#  if defined(__STDC_VERSION__)
+#    error Unexpected __STDC_VERSION__ definition
+#  endif
+#endif
+
+int main()
+{
+  return 0;
+}

Tests/CompileFeatures/default_dialect.cpp

@@ -0,0 +1,25 @@
+
+template<long l>
+struct Outputter;
+
+#if DEFAULT_CXX14
+#  if __cplusplus != 201402L
+Outputter<__cplusplus> o;
+#  endif
+#elif DEFAULT_CXX11
+#  if __cplusplus != 201103L
+Outputter<__cplusplus> o;
+#  endif
+#else
+#  if !DEFAULT_CXX98
+#    error Buildsystem error
+#  endif
+#  if __cplusplus != 199711L
+Outputter<__cplusplus> o;
+#  endif
+#endif
+
+int main()
+{
+  return 0;
+}

Tests/CompileFeatures/main.c

@@ -1,12 +0,0 @@
-
-int foo(int * restrict a, int * restrict b)
-{
-  (void)a;
-  (void)b;
-  return 0;
-}
-
-int main()
-{
-  return 0;
-}