Set _FOUND to trick ament_target_dependencies() for test

[sloretz]

blocks ament/ament_cmake#180

[jacobperron]

What call to ament_target_dependencies is causing the issue?

[sloretz]

What call to ament_target_dependencies is causing the issue?

@jacobperron this invokes the generator for the test messages, including the generator in this package.

rosidl/rosidl_generator_c/CMakeLists.txt

Line 107 in c0cb499

rosidl_generate_interfaces(${PROJECT_NAME}_interfaces ${message_files}

so this is called,

rosidl/rosidl_generator_c/cmake/rosidl_generator_c_generate_interfaces.cmake

Lines 127 to 129 in c0cb499

	ament_target_dependencies(${rosidl_generate_interfaces_TARGET}${_target_suffix}
	"rosidl_generator_c"
	"rosidl_typesupport_interface")

but rosidl_generator_c is still being built so it can't have been find_package()'d. These variables are tricking ament_target_dependencies() into thinking it was find_package()'d anyways.

rosidl/rosidl_generator_c/CMakeLists.txt

Lines 97 to 99 in c0cb499

	set(rosidl_generator_c_FOUND "1")
	set(rosidl_generator_c_DIR "${CMAKE_CURRENT_SOURCE_DIR}/cmake")
	set(rosidl_generator_c_LIBRARIES "${PROJECT_NAME}")