Made the supression of unused variables portable.

EIGEN_UNUSED is not supported on non GCC systems.

Eigen/src/Core/util/Macros.h

@@ -176,6 +176,9 @@
 #define EIGEN_UNUSED
 #endif
 
+// Suppresses 'unused variable' warnings.
+#define EIGEN_UNUSED_VARIABLE(var) (void)var;
+
 #if (defined __GNUC__)
 #define EIGEN_ASM_COMMENT(X)  asm("#"X)
 #else

Eigen/src/Core/util/Memory.h

@@ -218,7 +218,7 @@ inline void ei_aligned_free(void *ptr)
 **/
 inline void* ei_aligned_realloc(void *ptr, size_t new_size, size_t old_size)
 {
-  (void)old_size; // Suppress 'unused variable' warning. Seen in boost tee.
+  EIGEN_UNUSED_VARIABLE(old_size);
 
   void *result;
 #if !EIGEN_ALIGN