CMake: Support header-only NanoSVG library

Check if the NanoSVG target defines any library locations.
Add a private compile definition to wxcore, instead of adding another setup.h option.

build/cmake/lib/core/CMakeLists.txt

@@ -66,6 +66,9 @@ foreach(lib JPEG PNG TIFF NANOSVG)
         wx_lib_include_directories(wxcore ${${lib}_INCLUDE_DIR})
     endif()
 endforeach()
+if(wxUSE_NANOSVG STREQUAL "sys" AND wxUSE_NANOSVG_EXTERNAL_ENABLE_IMPL)
+    wx_lib_compile_definitions(wxcore wxUSE_NANOSVG_EXTERNAL_ENABLE_IMPL)
+endif()
 
 if(WIN32)
     wx_lib_link_libraries(wxcore PRIVATE winmm)