setup.py: detect noexec build fs issue, see borgbackup#8208

That "failed to map segment from shared object" error msg is not
very helpful. Add a hint that the filesystem needs to be +exec
(== not noexec mounted, like it might be the case for /tmp on
some systems).

setup.py

@@ -21,6 +21,8 @@
  # either there is no Cython installed or there is some issue with it.
  cythonize = None
  cythonize_import_error_msg = "ImportError: " + str(exc)
+ if "failed to map segment from shared object" in cythonize_import_error_msg:
+ cythonize_import_error_msg += " Check if the borg build uses a +exec filesystem."
 
 sys.path += [os.path.dirname(__file__)]