Set POSITION_INDEPENDENT_CODE ON for static library.

See discussion in #39.
diff --git a/src/CMakeLists.txt b/src/CMakeLists.txt
index eee9316..b40c1ac 100644
--- a/src/CMakeLists.txt
+++ b/src/CMakeLists.txt
@@ -88,7 +88,8 @@
   SOVERSION ${PROJECT_VERSION_MAJOR}.${PROJECT_VERSION_MINOR}.${PROJECT_VERSION_PATCH}
   VERSION ${PROJECT_VERSION})
 set_target_properties(${STATICLIBRARY} PROPERTIES
-  COMPILE_FLAGS -DCMARK_STATIC_DEFINE)
+  COMPILE_FLAGS -DCMARK_STATIC_DEFINE
+  POSITION_INDEPENDENT_CODE ON)
 
 if (MSVC)
   set_target_properties(${STATICLIBRARY} PROPERTIES