Fix potential invalid reset of CMAKE_FIND_LIBRARY_PREFIXES on MSVC.

- gflags_report_not_found() calls gflags_reset_find_library_prefix()
  and we only reset CMAKE_FIND_LIBRARY_PREFIXES when performing a manual
  search for gflags, but gflags_report_not_found() is also used
  before the manual search when searching for an exported gflags target.
- As such, it was possible that we could have cleared
  CMAKE_FIND_LIBRARY_PREFIXES rather than reset it if
  gflags_report_not_found() was invoked during the exported target
  search.
- This patch prevents this possibility by verifying that the cached
  version of CMAKE_FIND_LIBRARY_PREFIXES exists before updating it.

Change-Id: I07528ae5f197a366c7da342196b3e977f9a1fc93