Various improvements to the benchmarking support

* Units from <ratio> are no longer redeclared in our own namespace
* The default clock is `steady_clock`, not `high_resolution_clock`,
because, as HH says "high_resolution_clock is useless. If you want
measure the passing of time, use steady_clock. If you want user
friendly time, use system_clock".
* Benchmarking support is opt-in, not opt-out, to avoid the large
(~10%) compile time penalty.
* Benchmarking-related options in CLI are always present, to decrease
the amount of code that is only compiled conditionally and making
the whole shebang more maintainble.

include/reporters/catch_reporter_xml.cpp

@@ -219,7 +219,7 @@ namespace Catch {
         m_xml.endElement();
     }
 
-#ifndef CATCH_CONFIG_DISABLE_BENCHMARKING 
+#if defined(CATCH_CONFIG_ENABLE_BENCHMARKING)
     void XmlReporter::benchmarkStarting(BenchmarkInfo const &info) {
         m_xml.startElement("BenchmarkResults")
             .writeAttribute("name", info.name)
@@ -259,7 +259,7 @@ namespace Catch {
             writeAttribute("message", error);
         m_xml.endElement();
     }
-#endif // CATCH_CONFIG_DISABLE_BENCHMARKING
+#endif // CATCH_CONFIG_ENABLE_BENCHMARKING
 
     CATCH_REGISTER_REPORTER( "xml", XmlReporter )