COMP: Fix superbuild configuration error when OTB_WRAP_PYTHON is ON and USE_SYSTEM_SWIG is OFF

AFAIK, this configuration is not tested on the dashboard. Error before patch :

cmake -DCMAKE_CXX_FLAGS=-std=c++11 -DUSE_SYSTEM_SWIG=OFF -DOTB_WRAP_PYTHON=ON ~/dev/src/otb/SuperBuild/

Call Stack (most recent call first):
  CMake/SuperBuild_Macro.cmake:56 (include)
  CMake/External_otb.cmake:79 (ADDTO_DEPENDENCIES_IF_NOT_SYSTEM)
  CMakeLists.txt:326 (include)

CMake Error at CMake/External_swig.cmake:60 (ADD_SUPERBUILD_CONFIGURE_VAR):
  ADD_SUPERBUILD_CONFIGURE_VAR Macro invoked with incorrect arguments for
  macro named: ADD_SUPERBUILD_CONFIGURE_VAR
Call Stack (most recent call first):
  CMake/SuperBuild_Macro.cmake:56 (include)
  CMake/External_otb.cmake:79 (ADDTO_DEPENDENCIES_IF_NOT_SYSTEM)
  CMakeLists.txt:326 (include)

CMake Error at CMake/External_swig.cmake:61 (ADD_SUPERBUILD_CONFIGURE_VAR):
  ADD_SUPERBUILD_CONFIGURE_VAR Macro invoked with incorrect arguments for
  macro named: ADD_SUPERBUILD_CONFIGURE_VAR
Call Stack (most recent call first):
  CMake/SuperBuild_Macro.cmake:56 (include)
  CMake/External_otb.cmake:79 (ADDTO_DEPENDENCIES_IF_NOT_SYSTEM)
  CMakeLists.txt:326 (include)

SuperBuild/CMake/External_swig.cmake

@@ -57,8 +57,8 @@ else()
   # declare dependencies
   ADDTO_DEPENDENCIES_IF_NOT_SYSTEM(SWIG PCRE BOOST)
 
-  ADD_SUPERBUILD_CONFIGURE_VAR(PCRE_ROOT --with-pcre-prefix)
-  ADD_SUPERBUILD_CONFIGURE_VAR(BOOST_ROOT --with-boost)
+  ADD_SUPERBUILD_CONFIGURE_VAR(SWIG PCRE_ROOT --with-pcre-prefix)
+  ADD_SUPERBUILD_CONFIGURE_VAR(SWIG BOOST_ROOT --with-boost)
 
   ExternalProject_Add(SWIG
     PREFIX SWIG