Add SuiteSparse::IsConstrainedApproximateMinimumDegreeOrderingAvailable.

This allows checking for the availability of CAMD without depending
on a preprocessor symbol, which can be the source of subtle bugs
if the file defining that symbol was not included.

Change-Id: I6f1ef91a20baeecf68e8dd467ebe707dde8c5f79

internal/ceres/suitesparse.h

diff --git a/internal/ceres/suitesparse.h b/internal/ceres/suitesparse.h
index 0a7ea97..f27e338 100644
--- a/internal/ceres/suitesparse.h
+++ b/internal/ceres/suitesparse.h
@@ -283,9 +283,21 @@
 
 #else  // CERES_NO_SUITESPARSE
 
-class SuiteSparse {};
 typedef void cholmod_factor;
 
+class SuiteSparse {
+  // Defining this static function even when SuiteSparse is not
+  // available, allows client code to check for the presence of CAMD
+  // without checking for the absence of the CERES_NO_CAMD symbol.
+  //
+  // This is safer because the symbol maybe missing due to a user
+  // accidently not including suitesparse.h in their code when
+  // checking for the symbol.
+  static bool IsConstrainedApproximateMinimumDegreeOrderingAvailable() {
+    return false;
+  }
+};
+
 #endif  // CERES_NO_SUITESPARSE
 
 #endif  // CERES_INTERNAL_SUITESPARSE_H_