Pass CMAKE_GENERATOR_TOOLSET (cmake -T argument) to external projects

The dependencies introduced in #1620 cause an error when building natively on Windows, according to @ragundo. CMake is looking for build tools `v100`, which are part of VS2010 and won't work with DF/DFHack, even if they were installed.

Some of the Windows build scripts pass in `-T v140_xp` (from VS2015), so passing this option through to subprojects may help: https://cmake.org/cmake/help/latest/variable/CMAKE_GENERATOR_TOOLSET.html

depends/CMakeLists.txt

@@ -19,6 +19,10 @@ add_subdirectory(clsocket)
 ide_folder(clsocket "Depends")
 
 # assemble environment args to pass on to dependency projects
+if (NOT("${CMAKE_GENERATOR_TOOLSET}" STREQUAL ""))
+  list(APPEND CL_ARGS "-T" "${CMAKE_GENERATOR_TOOLSET}")
+endif()
+
 get_cmake_property(vars CACHE_VARIABLES)
 foreach(var ${vars})
   if(var MATCHES "^CMAKE_"