Review OpenIGTLink and ZMQ external project definition

Define INCLUDE_DIRS, LIBRARY_DIRS and FIND_PACKAGE_CMD so that the build
system can automatically handle the dependencies.

CMakeExternals/OpenIGTLink.cmake

@@ -10,6 +10,8 @@ IF(${add_project})
     MESSAGE(FATAL_ERROR "OpenIGTLink_DIR variable is defined but corresponds to non-existing directory")
   ENDIF()
   
+  SET(OpenIGTLink_enabling_variable OpenIGTLink_LIBRARIES)
+  
   SET(proj OpenIGTLink)
   SET(proj_DEPENDENCIES)
   
@@ -32,4 +34,9 @@ IF(${add_project})
   ELSE()
     ctkMacroEmptyExternalProject(${proj} "${proj_DEPENDENCIES}")
   ENDIF()
+  
+  SET(${OpenIGTLink_enabling_variable}_LIBRARY_DIRS OpenIGTLink_LIBRARY_DIRS)
+  SET(${OpenIGTLink_enabling_variable}_INCLUDE_DIRS OpenIGTLink_INCLUDE_DIRS)
+  SET(${OpenIGTLink_enabling_variable}_FIND_PACKAGE_CMD OpenIGTLink)
+  
 ENDIF()

CMakeExternals/ZMQ.cmake

@@ -38,6 +38,7 @@ IF(${add_project})
     ctkMacroEmptyExternalProject(${proj} "${proj_DEPENDENCIES}")
   ENDIF()
 
+  SET(${ZMQ_enabling_variable}_INCLUDE_DIRS ZMQ_LIBRARY_DIRS)
   SET(${ZMQ_enabling_variable}_INCLUDE_DIRS ZMQ_INCLUDE_DIRS)
   SET(${ZMQ_enabling_variable}_FIND_PACKAGE_CMD ZMQ)
 ENDIF()

Libs/Messaging/Core/CMakeLists.txt

@@ -1,26 +1,6 @@
 PROJECT(CTKMessagingCore)
 
 #
-# 3rd party dependencies
-#
-
-FIND_PACKAGE(ZMQ)
-# TODO In case ZMQ is passed directly using ZMQ_DIR, check expected options
-IF(NOT ZMQ_FOUND)
-  MESSAGE(FATAL_ERROR "error: ZMQ package is required to build ${PROJECT_NAME}")
-ENDIF()
-INCLUDE(${ZMQ_USE_FILE})
-
-
-#FIND_PACKAGE(OpenIGTLink)
-# TODO In case OpenIGTLink is passed directly using OpenIGTLink_DIR, check expected options
-#IF(NOT OpenIGTLink_FOUND)
-#  MESSAGE(FATAL_ERROR "error: OpenIGTLink package is required to build ${PROJECT_NAME}")
-#ENDIF()
-#INCLUDE(${OpenIGTLink_USE_FILE})
-
-
-#
 # See CTK/CMake/ctkMacroBuildLib.cmake for details
 #