FindwxWidgets: Do not use -isystem on OpenBSD (#13219)

Set wxWidgets_INCLUDE_DIRS_NO_SYSTEM on OpenBSD in addition to Apple to
prevent UsewxWidgets.cmake from using the include_directories SYSTEM
option.

Modules/FindwxWidgets.cmake

@@ -167,9 +167,9 @@ SET(wxWidgets_CXX_FLAGS    "")
 #   http://www.cmake.org/pipermail/cmake/2008-April/021115.html
 #   http://www.cmake.org/pipermail/cmake/2008-April/021146.html
 #
-IF(APPLE)
+IF(APPLE OR CMAKE_CXX_PLATFORM_ID MATCHES "OpenBSD")
   SET(wxWidgets_INCLUDE_DIRS_NO_SYSTEM 1)
-ENDIF(APPLE)
+ENDIF()
 
 # DEPRECATED: This is a patch to support the DEPRECATED use of
 # wxWidgets_USE_LIBS.