Tentative fix for msvc warnings.

See https://github.com/boostorg/regex/issues/61.
Adds warning test case.

test/test_warnings.cpp

@@ -0,0 +1,12 @@
+#ifdef _MSC_VER
+#pragma warning(disable:4820 4668)
+#endif
+
+#include <boost/regex.hpp>
+
+void test_proc()
+{
+   std::string text, re;
+   boost::regex exp(re);
+   regex_match(text, exp);
+}