Don't run tests on gcc 4 and earlier; not worth the pain.

test/mod_inverse_test.cpp

@@ -4,6 +4,7 @@
  *  Boost Software License, Version 1.0. (See accompanying file
  *  LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
  */
+#if __GNUC__ > 4
 #include <cassert>
 #include <boost/multiprecision/cpp_int.hpp>
 #include <boost/integer/common_factor.hpp>
@@ -57,3 +58,4 @@ int main()
 
     return 0;
 }
+#endif