cmake: turn on CMAKE_POSITION_INDEPENDENT_CODE for shared

This prevents an error while linking libbx.a into libbgfx.so on linux

CMakeLists.txt

@@ -41,6 +41,10 @@ set( BGFX_LIBRARY_TYPE     "STATIC" CACHE STRING "Linking type for library" )
 
 set_property( CACHE BGFX_LIBRARY_TYPE PROPERTY STRINGS STATIC SHARED )
 
+if( BGFX_LIBRARY_TYPE MATCHES "SHARED" )
+    set(CMAKE_POSITION_INDEPENDENT_CODE ON)
+endif()
+
 if( NOT BX_DIR )
 	set( BX_DIR "${CMAKE_CURRENT_SOURCE_DIR}/bx" CACHE STRING "Location of bx." )
 elseif( NOT IS_ABSOLUTE "${BX_DIR}")