BUG: avoid bug when PYTHON_VERSION_MAJOR is not defined

CMakeLists.txt

@@ -105,7 +105,7 @@ set ( Python_ADDITIONAL_VERSIONS "3;2" )
 set ( PythonInterp_FIND_REQUIRED ${OTB_WRAP_PYTHON} )
 set ( PythonLibs_FIND_REQUIRED ${OTB_WRAP_PYTHON} )
 find_package( PythonInterp )
-if ( OTB_WRAP_PYTHON AND ( ${PYTHON_VERSION_MAJOR} EQUAL 2 ) )
+if ( OTB_WRAP_PYTHON AND ( "x${PYTHON_VERSION_MAJOR}" STREQUAL "x2" ) )
     message (WARNING "Python3 not found. There is no longer support of \
     wrapping in python2 in OTB, but it can still be used.")
 endif()