Define WIN32_LEAN_AND_MEAN only if it has not been defined already.

In Chrome (and maybe other projects) WIN32_LEAN_AND_MEAN is defined
globally by the project environment already. This change makes the
behavior a little bit more conservative and should not have any impact on
existing projects using RE2. For Chrome it helps reducing the upstream
delta.

(Effectively merging pull request #68 by battre.)

Change-Id: Ib310665ae3e95ae3561d72ad3d973ab5839d36af
Reviewed-on: https://code-review.googlesource.com/3860
Reviewed-by: Paul Wankadia <junyer@google.com>
1 file changed