[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/IDConfigEigen.hpp

@@ -1,6 +1,7 @@
 ///@file Configuration for Inverse Dynamics Library with Eigen
 #ifndef INVDYNCONFIG_EIGEN_HPP_
 #define INVDYNCONFIG_EIGEN_HPP_
+#define btInverseDynamics btInverseDynamicsEigen
 #ifdef BT_USE_DOUBLE_PRECISION
 // choose double/single precision version
 typedef double idScalar;