[InverseDynamics] Add custom namespaces per configuration

BulletInverseDynamics can be configured to compile using Eigen
instead of Bullet's LinearMath.

Adds a preprocessor defined specific namespace for each
configuration to produce linker errors if incompatible
libraries are linked (eg, builtin core library and
utilities for bullet types).

src/BulletInverseDynamics/details/IDMatVec.hpp

@@ -4,6 +4,8 @@
 
 #include <cstdlib>
 
+#include "../IDConfig.hpp"
+
 namespace btInverseDynamics {
 class vec3;
 class vecx;