Fix: Find Qt5 (if enabled) in ElementalConfig.cmake

If Qt5 has been enabled during build, then e.g. target Qt5::Widgets is exported
as a install dependency via ElementalTargets.cmake. Thus ElementalConfig.cmake
must find Qt5 and import these Qt5 targets or else the user will get errors like:

CMake Error at CMakeLists.txt:... (add_executable):
  Target "..." links to target "Qt5::Widgets" but the target was not found.
  Perhaps a find_package() call is missing for an IMPORTED target, or an
  ALIAS target is missing?

cmake/configure_files/ElementalConfig.cmake.in

@@ -8,13 +8,21 @@ IF(@MPC_FOUND@)
    set(Elemental_INCLUDE_DIRS "${Elemental_INCLUDE_DIRS};@MPFR_INCLUDES@")
    set(Elemental_INCLUDE_DIRS "${Elemental_INCLUDE_DIRS};@GMP_INCLUDES@")
 ENDIF()
-set(Elemental_INCLUDE_DIRS
-  "${Elemental_INCLUDE_DIRS};@Qt5Widgets_INCLUDE_DIRS@")
+
+IF(@EL_HAVE_QT5@)
+  include(CMakeFindDependencyMacro)
+  set(QT_USE_IMPORTED_TARGETS TRUE)
+  find_dependency(Qt5Widgets)
+  set(Elemental_INCLUDE_DIRS
+    "${Elemental_INCLUDE_DIRS};@Qt5Widgets_INCLUDE_DIRS@")
+ENDIF()
 
 set(Elemental_COMPILE_FLAGS "@CXX_FLAGS@")
 set(Elemental_LINK_FLAGS "@EL_LINK_FLAGS@")
 
-set(Elemental_DEFINITIONS "@Qt5Widgets_DEFINITIONS@")
+IF(@EL_HAVE_QT5@)
+  set(Elemental_DEFINITIONS "@Qt5Widgets_DEFINITIONS@")
+ENDIF()
 
 # Our library dependencies (contains definitions for IMPORTED targets)
 include("${CMAKE_CURRENT_LIST_DIR}/ElementalTargets.cmake")