Compile with warnings on Unix.

The warnings got disabled at some point; this re-enables some of them, and
fixes some of the warnings.

Change-Id: I290a4fdfad18cea85e9177ba57744d97b6856bb2

internal/ceres/cxsparse.cc

diff --git a/internal/ceres/cxsparse.cc b/internal/ceres/cxsparse.cc
index ca36ce0..21c98e0 100644
--- a/internal/ceres/cxsparse.cc
+++ b/internal/ceres/cxsparse.cc
@@ -39,7 +39,7 @@
 namespace ceres {
 namespace internal {
 
-CXSparse::CXSparse() : scratch_size_(0), scratch_(NULL) {
+CXSparse::CXSparse() : scratch_(NULL), scratch_size_(0) {
 }
 
 CXSparse::~CXSparse() {