COMP: Improve support for building as a "Slicer Remote module"

This commit allows to remove the workaround added to the Slicer build system
to ensure OpenIGTLink was found before building OpenIGTLinkIF.

See https://www.slicer.org/wiki/Documentation/Nightly/Developers/Build_system/Remote_Module

Author: jcfr

CMakeLists.txt

@@ -6,8 +6,13 @@ set(MODULE_TITLE "OpenIGTLink I/F")
 string(TOUPPER ${MODULE_NAME} MODULE_NAME_UPPER)
 
 #-----------------------------------------------------------------------------
-find_package(Slicer REQUIRED)
-include(${Slicer_USE_FILE})
+if(Slicer_SOURCE_DIR)
+  find_package(OpenIGTLink REQUIRED)
+  include(${OpenIGTLink_USE_FILE})
+else()
+  find_package(Slicer REQUIRED)
+  include(${Slicer_USE_FILE})
+endif()
 
 # --------------------------------------------------------------------------
 # Check if version 2 protocol is available
@@ -94,7 +99,6 @@ slicerMacroBuildQtModule(
   RESOURCES ${MODULE_RESOURCES}
   )
 
-
 if(BUILD_TESTING)
   #add_subdirectory(Testing)
 endif()