Fix compiling with C++17 + Clang 5 + libstdc++ v5

This basically tests the combination where the compiler supports most
of C++17 but the library does not.

src/catch2/internal/catch_compiler_capabilities.hpp

@@ -222,7 +222,7 @@
   // Check if byte is available and usable
   #  if __has_include(<cstddef>) && defined(CATCH_CPP17_OR_GREATER)
   #    include <cstddef>
-  #    if __cpp_lib_byte > 0
+  #    if defined(__cpp_lib_byte) && (__cpp_lib_byte > 0)
   #      define CATCH_INTERNAL_CONFIG_CPP17_BYTE
   #    endif
   #  endif // __has_include(<cstddef>) && defined(CATCH_CPP17_OR_GREATER)

src/catch2/matchers/catch_matchers_templated.hpp

@@ -55,11 +55,11 @@ namespace Matchers {
             return arr;
         }
 
-        #ifdef CATCH_CPP17_OR_GREATER
+#if defined( __cpp_lib_logical_traits ) && __cpp_lib_logical_traits >= 201510
 
         using std::conjunction;
 
-        #else // CATCH_CPP17_OR_GREATER
+#else // __cpp_lib_logical_traits
 
         template<typename... Cond>
         struct conjunction : std::true_type {};
@@ -67,7 +67,7 @@ namespace Matchers {
         template<typename Cond, typename... Rest>
         struct conjunction<Cond, Rest...> : std::integral_constant<bool, Cond::value && conjunction<Rest...>::value> {};
 
-        #endif // CATCH_CPP17_OR_GREATER
+#endif // __cpp_lib_logical_traits
 
         template<typename T>
         using is_generic_matcher = std::is_base_of<