[concepts] Add vs2015 workaround.

Author: awulkiew

include/boost/geometry/geometries/concepts/check.hpp

@@ -219,7 +219,11 @@ struct checker<boost::variant<BOOST_VARIANT_ENUM_PARAMS(T)> const>
     \ingroup concepts
 */
 template <typename Geometry>
-constexpr inline void check()
+// workaround for VS2015
+#if !defined(_MSC_VER) || (_MSC_VER >= 1910)
+constexpr
+#endif
+inline void check()
 {
     detail::checker<Geometry> c;
     boost::ignore_unused(c);
@@ -232,7 +236,11 @@ constexpr inline void check()
     \ingroup concepts
 */
 template <typename Geometry1, typename Geometry2>
-constexpr inline void check_concepts_and_equal_dimensions()
+// workaround for VS2015
+#if !defined(_MSC_VER) || (_MSC_VER >= 1910)
+constexpr
+#endif
+inline void check_concepts_and_equal_dimensions()
 {
     check<Geometry1>();
     check<Geometry2>();