HPX fails to configure with -DHPX_WITH_TESTS=OFF

[khuck]

Expected Behavior

When configuring HPX for Phylanx builds, I disable the HPX tests with the -DHPX_WITH_TESTS=OFF flag. That causes cmake to fail.

Actual Behavior

CMake fails with several errors like:

CMake Error at cmake/HPX_AddPseudoDependencies.cmake:28 (add_dependencies):
  Cannot add target-level dependencies to non-existent target
  "tests.examples".

  The add_dependencies works for top-level logical targets created by the
  add_executable, add_library, or add_custom_target commands.  If you want to
  add file-level dependencies see the DEPENDS option of the add_custom_target
  and add_custom_command commands.
Call Stack (most recent call first):
  libs/cache/examples/CMakeLists.txt:8 (add_hpx_pseudo_dependencies)

Steps to Reproduce the Problem

1. cmake ... -DHPX_WITH_TESTS=OFF ...

Specifications

... Please describe your environment

• HPX Version: master, "stable" tag
• Platform (compiler, OS): linux, gcc 7.1