Fix ROS2 C++ build with proper ament CMake exports

This commit fixes the ROS2/colcon build integration by properly configuring
CMake exports for the ament ecosystem.

Changes:
- Add ament detection when not building as LF subproject
- Move install(EXPORT) from lib/CMakeLists.txt to root CMakeLists.txt
  (required for ament compatibility - must be alongside ament_package())
- Add NAMESPACE to export for proper imported target creation
- Add ament_export_targets(), ament_export_include_directories(), and
  ament_export_libraries() for full ament integration

The key insight is that ament's build system requires the export installation
and ament_package() to be in the same CMakeLists.txt file. Using namespaced
targets (reactor-cpp::reactor-cpp) ensures proper linkage through CMake's
imported target mechanism.

Related: lf-lang/lingua-franca#2580

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: lsk567

CMakeLists.txt

@@ -9,6 +9,13 @@ endif()
 
 project(${LIB_TARGET} LANGUAGES CXX VERSION 0.0.1)
 
+if (NOT DEFINED LF_REACTOR_CPP_SUFFIX)
+  find_package(ament_cmake QUIET)
+  if(ament_cmake_FOUND)
+    set(REACTOR_CPP_USE_AMENT ON)
+  endif()
+endif()
+
 # require C++17
 set(CMAKE_CXX_STANDARD 17)
 set(CMAKE_CXX_STANDARD_REQUIRED ON)
@@ -71,3 +78,17 @@ if (DEFINED LF_REACTOR_CPP_SUFFIX)
 else()
   install(DIRECTORY include/ DESTINATION "${CMAKE_INSTALL_INCLUDEDIR}")
 endif()
+
+if (REACTOR_CPP_USE_AMENT)
+  # Install the export file - must be in root CMakeLists.txt for ament compatibility
+  install(EXPORT ${LIB_TARGET} DESTINATION share/${LIB_TARGET}/cmake NAMESPACE ${LIB_TARGET}::)
+  ament_export_targets(${LIB_TARGET} HAS_LIBRARY_TARGET)
+  ament_export_include_directories(include)
+  ament_export_libraries(${LIB_TARGET})
+  ament_package()
+else()
+  # For non-ament builds, install export file from here
+  if(REACTOR_CPP_INSTALL)
+    install(EXPORT ${LIB_TARGET} DESTINATION share/${LIB_TARGET}/cmake)
+  endif()
+endif()

lib/CMakeLists.txt

@@ -61,12 +61,9 @@ if(REACTOR_CPP_INSTALL)
     install(FILES "${PROJECT_BINARY_DIR}/include/reactor-cpp/config.hh" DESTINATION "${CMAKE_INSTALL_INCLUDEDIR}/reactor-cpp")
   endif()
 
-  install(TARGETS ${LIB_TARGET} EXPORT ${LIB_TARGET}Config
+  install(TARGETS ${LIB_TARGET} EXPORT ${LIB_TARGET}
       ARCHIVE  DESTINATION "${CMAKE_INSTALL_LIBDIR}" OPTIONAL
       LIBRARY  DESTINATION "${CMAKE_INSTALL_LIBDIR}" OPTIONAL
       RUNTIME  DESTINATION "${CMAKE_INSTALL_BINDIR}" OPTIONAL)
-
-  install(EXPORT ${LIB_TARGET}Config DESTINATION share/${LIB_TARGET}/cmake)
-
-  export(TARGETS ${PROJECT_NAME} FILE ${LIB_TARGET}Config.cmake)
+  # Note: install(EXPORT ...) moved to root CMakeLists.txt for ament compatibility
 endif()

package.xml

@@ -6,17 +6,8 @@
  <description>A C++ reactor runtime</description>
  <maintainer email="christian.menard@tu-dresden.de">user</maintainer>
  <license>ISC License</license>
+ <buildtool_depend>ament_cmake</buildtool_depend>
  <export>
-   <build_type>cmake</build_type>
- </export>
-</package>
-<package format="3">
- <name>reactor-cpp-foo</name>
- <version>0.0.0</version>
- <description>A C++ reactor runtime</description>
- <maintainer email="christian.menard@tu-dresden.de">user</maintainer>
- <license>ISC License</license>
- <export>
-   <build_type>cmake</build_type>
+   <build_type>ament_cmake</build_type>
  </export>
 </package>