cmake: fix handling of BUILD_WITH_LIBPYTHON

Author: jcarpent

CMakeLists.txt

@@ -169,7 +169,8 @@ IF(BUILD_PYTHON_INTERFACE)
 
     MESSAGE(STATUS "Python compiler: ${_python_implementation_value}")
     IF(_python_implementation_value MATCHES "PyPy" )
-      MESSAGE(FATAL_ERROR "PyPy detected, therefore libpython is not available. Please set BUILD_WITH_LIBPYTHON to OFF.")
+      SET(BUILD_WITH_LIBPYTHON OFF)
+      MESSAGE(STATUS "PyPy detected, therefore libpython is not available and BUILD_WITH_LIBPYTHON set to OFF.")
     ENDIF()
   ENDIF()
 ELSE(BUILD_PYTHON_INTERFACE)