Prevent linking both static/shared libs

CMakeVOL.cmake

@@ -34,17 +34,14 @@ if (HDF5_VOL_ALLOW_EXTERNAL)
     message (FATAL_ERROR "HDF5_ALLOW_EXTERNAL_SUPPORT must be set to 'GIT' to allow building of external HDF5 VOL connectors")
   endif ()
 
-  set (HDF5_LIB_TARGETS "")
-  set (HDF5_HL_LIB_TARGETS "")
-
-  if (BUILD_STATIC_LIBS)
-    list(APPEND HDF5_LIB_TARGETS ${HDF5_LIB_TARGET})
-    list(APPEND HDF5_HL_LIB_TARGETS ${HDF5_HL_LIB_TARGET})
-  endif()
-
   if (BUILD_SHARED_LIBS)
-    list(APPEND HDF5_LIB_TARGETS ${HDF5_LIBSH_TARGET})
-    list(APPEND HDF5_HL_LIB_TARGETS ${HDF5_HL_LIBSH_TARGET})
+    set(HDF5_LIB_TARGETS ${HDF5_LIBSH_TARGET})
+    set(HDF5_HL_LIB_TARGETS ${HDF5_HL_LIBSH_TARGET})
+  else ()
+    if (BUILD_STATIC_LIBS)
+      set(HDF5_LIB_TARGETS ${HDF5_LIB_TARGET})
+      set(HDF5_HL_LIB_TARGETS ${HDF5_HL_LIB_TARGET})
+    endif()
   endif()
 
   # For compatibility, set some variables that projects would