Revert aa1f10f, which breaks internal OpenMP search; update add_hpx_c…

…onfig_test so

that it works properly for external CMake projects. Test-cased against trunk and the
external CMake example repo.

cmake/FindHPX_OpenMP.cmake

@@ -40,16 +40,11 @@ else()
 
     hpx_include(AddConfigTest)
 
-    set(cmake_root ${CMAKE_SOURCE_DIR})
-    if(HPX_EXTERNAL_CMAKE)
-      set(cmake_root ${HPX_ROOT})
-    endif()
-
     foreach(flag_candidate ${flag_candidates}) 
       hpx_info("find_package.openmp" "Trying flag ${flag_candidate}.")
 
       add_hpx_config_test("openmp" OPENMP_FOUND LANGUAGE CXX
-        SOURCE ${cmake_root}/cmake/tests/openmp.cpp
+        SOURCE cmake/tests/openmp.cpp
         FLAGS ${flag_candidate} FILE)
 
       if(OPENMP_FOUND)

cmake/HPX_AddConfigTest.cmake

@@ -34,7 +34,12 @@ macro(add_hpx_config_test name variable)
       if(${name}_ROOT)
         set(test_source "${${name}_ROOT}/share/hpx-${HPX_VERSION}/${${name}_SOURCE}")
       else()
-        set(test_source "${hpx_SOURCE_DIR}/${${name}_SOURCE}")
+        set(cmake_root ${hpx_SOURCE_DIR})
+        if(HPX_EXTERNAL_CMAKE)
+          set(unresolved_root ${HPX_ROOT}/share/hpx)
+          get_filename_component(cmake_root ${unresolved_root} REALPATH)
+        endif()
+        set(test_source "${cmake_root}/${${name}_SOURCE}")
       endif()
     else()
       set(test_source