Minor tweaks to boost library detection

- Change default WITH_BOOST path to reside inside source tree
  to avoid permission problems or accidentally overwriting
  directories outside source tree.
- If system Boost installation is not found, optionally use
  header only unit test framework from location specified by
  WITH_BOOST.
- Fix extra-semi warning in provider_options.hpp.

include/wsrep/provider_options.hpp

@@ -57,7 +57,7 @@ namespace wsrep
         class option_value
         {
         public:
-            virtual ~option_value(){};
+            virtual ~option_value(){}
             virtual const char* as_string() const = 0;
             virtual const void* get_ptr() const = 0;
         };