Compatibility with CMake's imported targets LAPACK::LAPACK

[leha-bot]

The upstream CMake has some machinery to find the LAPACK library and it defines the LAPACK::LAPACK imported target.
It would be very convenient if the lapack itself make this imported target
See upstream documentation for package API details and
see this new fancy guide about importing and exporting targets

Thanks for your attention!

[leha-bot]

I also think that it could be implemented via modifying the (lapack-config-install.cmake.in)[https://github.com/Reference-LAPACK/lapack/blob/master/CMAKE/lapack-config-install.cmake.in] by adding this snippet:

add_library(LAPACK::LAPACK UNKNOWN IMPORTED)
target_link_libraries(LAPACK::LAPACK PUBLIC lapack)

but I'm not exactly sure about it. I'll try to check it and make PR if you will be interested in it

[thijssteel]

Sounds interesting, some notes though:

• this is the reference implementation of LAPACK, it is usually more appropriate to use other implementations.
• most of the contributers here have a more mathematical background, so we wouldn't really be able to keep that config up to date if there are changes upstream

[christoph-conrads]

• this is the reference implementation of LAPACK, it is usually more appropriate to use other implementations.

Usually but in 2016 for example there was still no 2-by-1 CS decomposition in Intel MKL.

• most of the contributers here have a more mathematical background, so we wouldn't really be able to keep that config up to date if there are changes upstream

CMake can automatically generate the target export. There is hardly any need to maintain it but #488 should be resolved before.

From the top of my head:

include(GNUInstallDirs) # avoid having to bother with `lib`, `lib64` directories
option(BUILD_SHARED_LIBS "Build shared libraries" CACHE ON)
add_library(blas sgemm.f)
add_library(lapack sggqrcs.f)
target_link_libraries(lapack blas)
install(
  TARGET blas
  LIBRARY DESTINATION ${CMAKE_INSTALL_LIBDIR}
  EXPORT blas-export
)
install(
  TARGET lapack
  LIBRARY DESTINATION ${CMAKE_INSTALL_LIBDIR}
  EXPORT lapack-export
)
install(
  EXPORT blas-export lapack-export
  DESTINATION ${CMAKE_INSTALL_LIBDIR}/${PROJECT_NAME}-${PROJECTION_VERSION_MAJOR}.${PROJECT_VERSION_MINOR}