Add POSITION_INDEPENDENT_CODE option

Allow to use the static library in other dynamic librairies. This is a standard cmake option https://cmake.org/cmake/help/v3.11/prop_tgt/POSITION_INDEPENDENT_CODE.html#prop_tgt:POSITION_INDEPENDENT_CODE

Author: BotellaA

CMakeLists.txt

@@ -85,6 +85,7 @@ set_target_properties(pugixml-shared pugixml-static pugixml
   PROPERTIES
     MSVC_RUNTIME_LIBRARY ${CMAKE_MSVC_RUNTIME_LIBRARY}
     EXCLUDE_FROM_ALL ON
+    POSITION_INDEPENDENT_CODE ON
     SOVERSION ${PROJECT_VERSION_MAJOR}
     VERSION ${PROJECT_VERSION})