BUG fix issue 1656 related to eratic behavior of cmake configuration

Modules/Wrappers/SWIG/otb-module-init.cmake

@@ -50,13 +50,16 @@ if ( OTB_WRAP_PYTHON )
 endif()
 
 macro(swap_cache_variable var1 var2)
-  set(_backup ${${var1}})
-  get_property(_var1_type CACHE ${var1} PROPERTY TYPE)
-  get_property(_var1_help CACHE ${var1} PROPERTY HELPSTRING)
-  get_property(_var2_type CACHE ${var2} PROPERTY TYPE)
-  get_property(_var2_help CACHE ${var2} PROPERTY HELPSTRING)
-  set(${var1} ${${var2}} CACHE ${_var1_type} "${_var1_help}" FORCE)
-  set(${var2} ${_backup} CACHE ${_var2_type} "${_var2_help}" FORCE)
+  # Only swap variable if var1 exists in the cmake cache
+  if (${var1})
+    set(_backup ${${var1}})
+    get_property(_var1_type CACHE ${var1} PROPERTY TYPE)
+    get_property(_var1_help CACHE ${var1} PROPERTY HELPSTRING)
+    get_property(_var2_type CACHE ${var2} PROPERTY TYPE)
+    get_property(_var2_help CACHE ${var2} PROPERTY HELPSTRING)
+    set(${var1} ${${var2}} CACHE ${_var1_type} "${_var1_help}" FORCE)
+    set(${var2} ${_backup} CACHE ${_var2_type} "${_var2_help}" FORCE)
+  endif()
 endmacro()
 
 macro(swap_variable var1 var2)