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/sparse_normal_cholesky_solver.h b/internal/ceres/sparse_normal_cholesky_solver.h
index 8d48096..ebb32e6 100644
--- a/internal/ceres/sparse_normal_cholesky_solver.h
+++ b/internal/ceres/sparse_normal_cholesky_solver.h
@@ -34,6 +34,8 @@
 #ifndef CERES_INTERNAL_SPARSE_NORMAL_CHOLESKY_SOLVER_H_
 #define CERES_INTERNAL_SPARSE_NORMAL_CHOLESKY_SOLVER_H_
 
+#if !defined(CERES_NO_SUITESPARSE) || !defined(CERES_NO_CXSPARSE)
+
 #include "ceres/cxsparse.h"
 #include "ceres/internal/macros.h"
 #include "ceres/linear_solver.h"
@@ -90,4 +92,5 @@
 }  // namespace internal
 }  // namespace ceres
 
+#endif  // !defined(CERES_NO_SUITESPARSE) || !defined(CERES_NO_CXSPARSE)
 #endif  // CERES_INTERNAL_SPARSE_NORMAL_CHOLESKY_SOLVER_H_