Modify googlehash use to account for namespace issues.

The namespace declaration for googlehash is a configurable macro that
can be disabled.  In particular, it is disabled within google, causing
compile errors since `dense_hash_map`/`sparse_hash_map` are then in
the global namespace instead of in `::google`.

Here we play a bit of gynastics to allow for both `google::*_hash_map`
and `*_hash_map`, while limiting namespace polution.  Symbols within
the `::google` namespace are imported into `Eigen::google`.

We also remove checks based on `_SPARSE_HASH_MAP_H_`, as this is
fragile, and instead require `EIGEN_GOOGLEHASH_SUPPORT` to be
defined.

bench/sparse_randomsetter.cpp

@@ -1,6 +1,7 @@
 
 #define NOGMM
 #define NOMTL
+#define EIGEN_GOOGLEHASH_SUPPORT 1
 
 #include <map>
 #include <ext/hash_map>