Install CMake config to LIBDIR by default, to correctly support multiarch

[ferdnyc]

Using share/cmake/${PROJECT_NAME} as DESTINATION for installing ZeroMQConfig.cmake et al works for arch-independent configs (like cppzmq's header-only package), but is wrong for arch-dependent packages when installed on multiarch systems.

The ...Config.cmake file is the library interface definition, and in arch-dependent multiarch builds is specific to the architecture currently being built. Each build of a library should have its interface installed below the arch-dependent LIBDIR.

This PR sets the default ZEROMQ_CMAKECONFIG_INSTALL_DIR to "${CMAKE_INSTALL_LIBDIR}/cmake/${PROJECT_NAME}", using the GNUInstallDirs ${CMAKE_INSTALL_LIBDIR} variable to target the correct arch-dependent dir.

See e.g. Daniel Pfeifer's "Effective CMake" (PDF of slides from CPPNow 2017), though he doesn't use GNUInstallDirs:

Export your library interface!

include(CMakePackageConfigHelpers)
write_basic_package_version_file("FooConfigVersion.cmake"
  VERSION ${Foo_VERSION}
  COMPATIBILITY SameMajorVersion
  )
install(FILES "FooConfig.cmake" "FooConfigVersion.cmake"
  DESTINATION lib/cmake/Foo
  )

Or Pablo Arias' "It's Time To Do CMake Right" post, which does:

install(EXPORT jsonutils-targets
  FILE
    JSONUtilsTargets.cmake
  NAMESPACE
    JSONUtils::
  DESTINATION
    ${CMAKE_INSTALL_LIBDIR}/cmake/JSONUtils
)

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