This was added by commit v2.8.0~2292 (... Set variable
wxWidgets_INCLUDE_DIRS_NO_SYSTEM on the Mac ..., 2008-04-16) and updated
by commit v2.8.9~183^2 (FindwxWidgets: Do not use -isystem on OpenBSD,
2012-05-14).  Since the underlying cause was never investigated fully we
do not know the conditions under which -isystem breaks wxWidgets, but
suppressing -isystem is problematic for users that do not want to see
warnings in wxWidgets headers.  Simply drop the special case for now so
we can see whether anyone hits the problem again, at which point it can
be investigated in more detail.

Reported-by: Simon Wells <swel024@gmail.com>