[buildsystem] Don't rely on a define from a header to detect that header

We can't use a define from a header (__cpp_lib_experimental_detect in
this case) to detect whether we should include the header. Without
C++17 __has_include, CMake will have to do the work after all.

cmake/modules/CheckCXXFeatures.cmake

@@ -31,6 +31,7 @@
 
 include(CMakePushCheckState)
 include(CheckCXXCompilerFlag)
+include(CheckIncludeFileCXX)
 include(CheckCXXSourceCompiles)
 
 # C++ standard versions that this test knows about
@@ -462,3 +463,9 @@ check_cxx_source_compiles("
   }
 " DUNE_SUPPORTS_CXX_THROW_IN_CONSTEXPR
   )
+
+# Check whether we have <experimental/type_traits> (for is_detected et. al.)
+check_include_file_cxx(
+  experimental/type_traits
+  DUNE_HAVE_HEADER_EXPERIMENTAL_TYPE_TRAITS
+  )

config.h.cmake

@@ -32,6 +32,9 @@
 /* does the compiler support conditionally throwing exceptions in constexpr context? */
 #cmakedefine DUNE_SUPPORTS_CXX_THROW_IN_CONSTEXPR 1
 
+/* does the standard library provide <experimental/type_traits> ? */
+#cmakedefine DUNE_HAVE_HEADER_EXPERIMENTAL_TYPE_TRAITS 1
+
 /* Define if you have a BLAS library. */
 #cmakedefine HAVE_BLAS 1
 

dune/common/std/type_traits.hh

@@ -7,7 +7,7 @@
 #include <dune/common/typetraits.hh>
 #include <dune/common/typeutilities.hh>
 
-#if __cpp_lib_experimental_detect >= 201505
+#if DUNE_HAVE_HEADER_EXPERIMENTAL_TYPE_TRAITS
 #include <experimental/type_traits>
 #endif