Add support for imported targets

[jcfr]

• Add CMake property target for disabling the use of CMAKE_<Lang>_IMPLICIT_LINK_* variables.

This is relevant when the project enables Fortran language and its CMake_Fortran_COMPILER_ID is different from the one of targets imported via find_package(Foo)

Explicitly updating the Foo targets setting IMPLICIT_LINK_<LANG>_ENABLED to 0 will ensure only the <Fortran_IMPORTED_TARGET_NAMESPACE>_Fortran_<Fortran_COMPILER_ID>_SUPPORT_LIBRARIES are linked.

Within property like IMPLICIT_LINK_Fortran_ENABLED, it looks like the same effect can be achieved removing Fortran from the IMPORTED_LINK_INTERFACE_LANGUAGES_RELEASE properties but this cumbersome:

set_target_properties(blas PROPERTIES IMPORTED_LINK_INTERFACE_LANGUAGES_RELEASE "")
set_target_properties(lapack PROPERTIES IMPORTED_LINK_INTERFACE_LANGUAGES_RELEASE "")

Ideally the foo-config.cmake.in would look like this:

[...]

include("${_Foo_SELF_DIR}/foo-targets.cmake")

# Hint for project building against Foo
set(Foo_Fortran_COMPILER_ID "@CMAKE_Fortran_COMPILER_ID@")

if("SupportLibraries" IN_LIST Foo_FIND_COMPONENTS)
  include(CMakeFindDependencyMacro)

  set(Fortran_COMPILER_ID ${Foo_Fortran_COMPILER_ID})
  set(Fortran_IMPORTED_TARGET_NAMESPACE "Foo")

  find_dependency(Fortran)

  target_link_libraries(Foo INTERFACE  ${Foo_Fortran_${Foo_Fortran_COMPILER_ID}_SUPPORT_LIBRARIES})

  set_target_properties(${Fortran_IMPORTED_TARGET_NAMESPACE}::Fortran_${_id}_${_lib} 
    PROPERTIES IMPLICIT_LINK_ENABLED 0)

  # cleanup
  unset(Fortran_COMPILER_ID)
  unset(Fortran_IMPORTED_TARGET_NAMESPACE)
endif()