Fix compiling v2.x 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.

include/internal/catch_compiler_capabilities.h

@@ -234,7 +234,7 @@
   // 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)