Include also environment LD_LIBRARY_PATH in gtk-doc-scangobj call

Thus it can be used to influence where the needed libraries are
looked for from the outside.

cmake/modules/GtkDoc.cmake

@@ -133,7 +133,7 @@ macro(add_gtkdoc _module _namespace _deprecated_guards _srcdirsvar _depsvar _ign
 		endif(TARGET ${opt})
 	endforeach(opt)
 
-	set(_scangobj_prefix ${CMAKE_COMMAND} -E env LD_LIBRARY_PATH="${_scangobj_ld_lib_dirs}:${LIB_INSTALL_DIR}")
+	set(_scangobj_prefix ${CMAKE_COMMAND} -E env LD_LIBRARY_PATH="${_scangobj_ld_lib_dirs}:${LIB_INSTALL_DIR}:$ENV{LD_LIBRARY_PATH}")
 
 	if(NOT (_scangobj_cflags STREQUAL ""))
 		set(_scangobj_cflags --cflags "${_scangobj_cflags}")