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

internal/ceres/schur_complement_solver.cc

@@ -141,6 +141,7 @@ bool DenseSchurComplementSolver::SolveReducedLinearSystem(double* solution) {
   return true;
 }
 
+#if !defined(CERES_NO_SUITESPARSE) || !defined(CERES_NO_CXSPARE)
 
 SparseSchurComplementSolver::SparseSchurComplementSolver(
     const LinearSolver::Options& options)
@@ -365,5 +366,6 @@ bool SparseSchurComplementSolver::SolveReducedLinearSystemUsingCXSparse(
 }
 #endif  // CERES_NO_CXPARSE
 
+#endif // !defined(CERES_NO_SUITESPARSE) || !defined(CERES_NO_CXSPARE)
 }  // namespace internal
 }  // namespace ceres