catkin: how to set up build dependencies of two packages correctly?

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Hello dear ROS-community!

I am currently struggeling with the setup of "CMakeLists.txt" and "package.xml" of two selfmade catkin packages. To my eyes it does not seem to be a very complicated situation, but somehow I am not able to make it work...

Here is the situation: Package A builds a c++ library, Package B uses and links against this library.

From my understanding, the CMakeLists.txt of Package A should therefore look something like this:

CMakeLists.txt Package A:

cmake_minimum_required(VERSION 2.8.3)
project(Package_A)
find_package(catkin REQUIRED COMPONENTS
 roscpp
 rospy
)

catkin_package(
INCLUDE_DIRS include 
LIBRARIES libmylibrary.so 
CATKIN_DEPENDS roscpp rospy
# DEPENDS system_lib
)

Package.xml Package A:

<name>Package_A</name>
<version>0.0.0</version>
<description>The A package</description>
<maintainer email="foo@bar.de">foobar</maintainer>
<license>TODO</license>
<buildtool_depend>catkin</buildtool_depend>
<build_depend>roscpp</build_depend>
<build_depend>rospy</build_depend>
<run_depend>roscpp</run_depend>
<run_depend>rospy</run_depend>

So now here comes package B, which needs the library of Package A:

CMakeLists.txt Package B:

cmake_minimum_required(VERSION 2.8.3)
project(Package_B)
find_package(catkin REQUIRED COMPONENTS
 roscpp
 rospy
 Package_A
)

catkin_package(
# INCLUDE_DIRS include 
# LIBRARIES libmylibrary.so 
CATKIN_DEPENDS Package_A
# DEPENDS system_lib
)

Package.xml Package B:

<name>Package_B</name>
<version>0.0.0</version>
<description>The B package</description>
<maintainer email="foo@bar.de">foobar</maintainer>
<license>TODO</license>
<buildtool_depend>catkin</buildtool_depend>
<build_depend>roscpp</build_depend>
<build_depend>rospy</build_depend>
<build_depend>Package_A</build_depend>
<run_depend>roscpp</run_depend>
<run_depend>rospy</run_depend>
<run_depend>Package_A</run_depend>

In this configuration, it is not possible to run "catkin_make", it will stop with the following error:
Project 'Package_B' tried to find library 'libmylibrary.so'. The library is neither a target nor built/installed properly. Did you compile project 'Package_A'?

Why doesn't catkin see the dependency and build the library first, so that the other package can find it?

My only workaround at the moment is to remove the dependencies, build the library stand-alone, put the dependency back in and then build the rest. What am I doing wrong?

Thanks for your answers