Fix vs2019 & namespace for install targets (#349)

* Change export namespace from `s2` to `s2::`
* Add absl dependency to Config.cmake.in
* Use C++14 form of `static_assert`

CMakeLists.txt

@@ -441,7 +441,7 @@ install(TARGETS s2 EXPORT s2Targets)
 # that they can referenced  by downstream projects as `s2::s2` after a
 # successful `find_package` call.
 install(EXPORT s2Targets
-        NAMESPACE s2
+        NAMESPACE s2::
         FILE s2Targets.cmake
         DESTINATION share/s2/)
 

Config.cmake.in

@@ -1,3 +1,6 @@
 @PACKAGE_INIT@
 
+include(CMakeFindDependencyMacro)
+find_dependency(absl CONFIG)
+
 include("${CMAKE_CURRENT_LIST_DIR}/s2Targets.cmake")

src/s2/s2lax_polygon_shape.cc

@@ -50,7 +50,7 @@ namespace {
 template <typename T>
 unique_ptr<T> make_unique_for_overwrite(size_t n) {
   // We only need to support this one variant.
-  static_assert(std::is_array<T>::value);
+  static_assert(std::is_array<T>::value, "T must be an array type");
   return unique_ptr<T>(new typename absl::remove_extent_t<T>[n]);
 }
 }  // namespace