Apply this patch, and mongod will no longer compile on Windows/Linux because other files rely on using std::string in the s2 header files.

s2dep.patch

From 89467d3f94472172bc2eca440a8b6144be2b14bb Mon Sep 17 00:00:00 2001
From: Kevin Albertson <kevin.albertson@10gen.com>
Date: Wed, 15 Jul 2015 22:26:53 -0400
Subject: [PATCH] Where things went wrong between me and scons

---
 src/mongo/db/index/s2_common.h | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/src/mongo/db/index/s2_common.h b/src/mongo/db/index/s2_common.h
index fad4087..4e4ee13 100644
--- a/src/mongo/db/index/s2_common.h
+++ b/src/mongo/db/index/s2_common.h
@@ -29,10 +29,14 @@
 #include "mongo/db/jsobj.h"
 #include "mongo/db/geo/geoconstants.h"
 #include "mongo/db/geo/s2.h"
+/*
 #include "third_party/s2/s2cell.h"
 #include "third_party/s2/s2polyline.h"
 #include "third_party/s2/s2polygon.h"
 #include "third_party/s2/s2regioncoverer.h"
+*/
+
+class S2RegionCoverer;
 
 #pragma once
 
@@ -79,10 +83,12 @@ struct S2IndexingParams {
     }
 
     void configureCoverer(S2RegionCoverer* coverer) const {
+        /*
         coverer->set_min_level(coarsestIndexedLevel);
         coverer->set_max_level(finestIndexedLevel);
         // This is advisory; the two above are strict.
         coverer->set_max_cells(maxCellsInCovering);
+        */
     }
 };
 
-- 
2.3.2 (Apple Git-55)