[CMake] Fixed a compile warning.

MathLib/CMakeLists.txt

@@ -6,7 +6,9 @@ APPEND_SOURCE_FILES(SOURCES InterpolationAlgorithms)
 APPEND_SOURCE_FILES(SOURCES Integration)
 APPEND_SOURCE_FILES(SOURCES LinAlg)
 APPEND_SOURCE_FILES(SOURCES LinAlg/Dense)
-APPEND_SOURCE_FILES(SOURCES ODE)
+if(CVODE_FOUND)
+    APPEND_SOURCE_FILES(SOURCES ODE)
+endif()
 APPEND_SOURCE_FILES(SOURCES Nonlinear)
 
 if(OGS_USE_EIGEN)

MathLib/ODE/CVodeSolver.cpp

@@ -6,9 +6,6 @@
  *              http://www.opengeosys.org/project/license
  *
  */
-
-#ifdef CVODE_FOUND
-
 #include "CVodeSolver.h"
 
 #include <cassert>
@@ -362,5 +359,3 @@ CVodeSolver::~CVodeSolver() = default;
 
 }  // namespace ODE
 }  // namespace MathLib
-
-#endif  // CVODE_FOUND