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/line_search_direction.cc b/internal/ceres/line_search_direction.cc
index 1fc4de5..b8b582c 100644
--- a/internal/ceres/line_search_direction.cc
+++ b/internal/ceres/line_search_direction.cc
@@ -28,6 +28,8 @@
 //
 // Author: sameeragarwal@google.com (Sameer Agarwal)
 
+#ifndef CERES_NO_LINE_SEARCH_MINIMIZER
+
 #include "ceres/line_search_direction.h"
 #include "ceres/line_search_minimizer.h"
 #include "ceres/low_rank_inverse_hessian.h"
@@ -143,3 +145,5 @@
 
 }  // namespace internal
 }  // namespace ceres
+
+#endif  // CERES_NO_LINE_SEARCH_MINIMIZER