fatal error: 'google/sparse_hash_map' file not found

I have installed sparsehash without any troubles. While trying to build fast_align getting the following error:
```
error:/Users/shreesh/Project/fast_align-master/src/hashtables.h:5:10: fatal error: 'google/sparse_hash_map' file
not found
#include
^~~~~~~~~~~~~~~~~~~~~~~~
1 error generated.
make[2]: *** [CMakeFiles/fast_align.dir/src/fast_align.cc.o] Error 1
make[1]: *** [CMakeFiles/fast_align.dir/all] Error 2
make: *** [all] Error 2
```
I tried manually changing the paths in hashtables.h but still couldn't get it to work.

Start with src/hashtables.h and reproduce the failing make/CMake build to confirm how the google/sparse_hash_map include is resolved. Inspect the build's sparsehash dependency and include-path setup; done means fast_align builds without the missing-header error.

Written by the indexing model from the issue text.