Modularize the build.

1. Add -DLINE_SEARCH_MINIMIZER to CMake to make the line search
   minimizer optional.
2. Better handling of -DSUITESPARSE/-DCXSPARSE in top level cmake
   file.
3. Disable code which will never be used if SuiteSparse and/or
   CXSparse is not available.
4. Update build docs.
5. Update jni/Android.mk
6. Minor lint cleanup from William Rucklidge.

Change-Id: If60460a858000df82faed7a6bb056dd2bfdde562
diff --git a/internal/ceres/canonical_views_clustering.cc b/internal/ceres/canonical_views_clustering.cc
index d0dc1e6..6531945 100644
--- a/internal/ceres/canonical_views_clustering.cc
+++ b/internal/ceres/canonical_views_clustering.cc
@@ -29,6 +29,8 @@
 // Author: David Gallup (dgallup@google.com)
 //         Sameer Agarwal (sameeragarwal@google.com)
 
+#ifndef CERES_NO_SUITESPARSE
+
 #include "ceres/canonical_views_clustering.h"
 
 #include "ceres/collections_port.h"
@@ -236,3 +238,5 @@
 
 }  // namespace internal
 }  // namespace ceres
+
+#endif  // CERES_NO_SUITESPARSE