--remove bullet physics as separate library and just include in physics

src/esp/physics/CMakeLists.txt

@@ -12,8 +12,8 @@ configure_file(
   ${CMAKE_CURRENT_SOURCE_DIR}/configure.h.cmake ${CMAKE_CURRENT_BINARY_DIR}/configure.h
 )
 
-add_library(
-  physics STATIC
+set(
+  physics_SOURCES
   ArticulatedLink.h
   ArticulatedObject.h
   CollisionGroupHelper.cpp
@@ -41,10 +41,38 @@ add_library(
 )
 
 if(BUILD_WITH_BULLET)
-  add_subdirectory(bullet)
 
-  target_link_libraries(physics PUBLIC bulletphysics)
-endif()
+  find_package(MagnumIntegration REQUIRED Bullet)
+  find_package(Bullet REQUIRED Dynamics)
+  # Add bullet sources
+  set(
+    physics_SOURCES
+    ${physics_SOURCES}
+    bullet/BulletArticulatedLink.h
+    bullet/BulletArticulatedObject.cpp
+    bullet/BulletArticulatedObject.h
+    bullet/BulletBase.cpp
+    bullet/BulletBase.h
+    bullet/BulletCollisionHelper.cpp
+    bullet/BulletCollisionHelper.h
+    bullet/BulletPhysicsManager.cpp
+    bullet/BulletPhysicsManager.h
+    bullet/BulletRigidObject.cpp
+    bullet/BulletRigidObject.h
+    bullet/BulletRigidStage.cpp
+    bullet/BulletRigidStage.h
+    bullet/BulletURDFImporter.cpp
+    bullet/BulletURDFImporter.h
+    bullet/objectWrappers/ManagedBulletArticulatedObject.h
+    bullet/objectWrappers/ManagedBulletRigidObject.h
+  )
+
+endif() #BUILD_WITH_BULLET
+
+add_library(
+  physics STATIC
+  ${physics_SOURCES}
+)
 
 target_link_libraries(
   physics
@@ -58,4 +86,8 @@ target_link_libraries(
          MagnumPlugins::StbImageConverter
 )
 
+if(BUILD_WITH_BULLET)
+  target_link_libraries(physics PUBLIC MagnumIntegration::Bullet Bullet::Dynamics)
+endif() #BUILD_WITH_BULLET
+
 set_directory_properties(PROPERTIES CORRADE_USE_PEDANTIC_FLAGS ON)