fix build to work when cmake does not fiind Bullet

libraries/entities/src/EntityMotionState.cpp

@@ -9,6 +9,7 @@
 //  See the accompanying file LICENSE or http://www.apache.org/licenses/LICENSE-2.0.html
 //
 
+#ifdef USE_BULLET_PHYSICS
 #include <BulletUtil.h>
 
 #include "EntityItem.h"
@@ -70,3 +71,4 @@ void EntityMotionState::computeShapeInfo(ShapeInfo& info) {
     info.setBox(bulletHalfExtents);
 }
 
+#endif // USE_BULLET_PHYSICS

libraries/physics/src/BulletUtil.h

@@ -12,6 +12,8 @@
 #ifndef hifi_BulletUtil_h
 #define hifi_BulletUtil_h
 
+#ifdef USE_BULLET_PHYSICS
+
 #include <btBulletDynamicsCommon.h>
 #include <glm/glm.hpp>
 #include <glm/gtc/quaternion.hpp>
@@ -37,4 +39,5 @@ inline void glmToBullet(const glm::quat& g, btQuaternion& b) {
     b = btQuaternion(g.x, g.y, g.z, g.w);
 }
 
+#endif // USE_BULLET_PHYSICS
 #endif // hifi_BulletUtil_h

libraries/physics/src/PhysicsWorld.cpp

@@ -10,6 +10,7 @@
 //
 
 #include "PhysicsWorld.h"
+#ifdef USE_BULLET_PHYSICS
 
 PhysicsWorld::~PhysicsWorld() {
 }
@@ -131,3 +132,5 @@ bool PhysicsWorld::updateEntityMassProperties(CustomMotionState* motionState, fl
     assert(motionState);
     return false;
 }
+
+#endif // USE_BULLET_PHYSICS

tests/physics/src/BulletUtilTests.cpp

@@ -16,6 +16,7 @@
 
 #include "BulletUtilTests.h"
 
+#ifdef USE_BULLET_PHYSICS
 void BulletUtilTests::fromBulletToGLM() {
     btVector3 bV(1.23f, 4.56f, 7.89f);
     glm::vec3 gV;
@@ -105,3 +106,13 @@ void BulletUtilTests::runAllTests() {
     fromGLMToBullet();
 }
 
+#else // USE_BULLET_PHYSICS
+void BulletUtilTests::fromBulletToGLM() {
+}
+
+void BulletUtilTests::fromGLMToBullet() {
+}
+
+void BulletUtilTests::runAllTests() {
+}
+#endif // USE_BULLET_PHYSICS