Work around using Python in Cygwin AppVeyor build

This doesn't work any longer since Cygwin Python 2.7.14 update, see
https://cygwin.com/ml/cygwin/2017-11/msg00053.html

Avoid using it by preventing include/wx/stc/stc.h file from being
updated, this shouldn't be necessary anyhow as the version in git should
be already up-to-date.

build/tools/appveyor.bat

@@ -41,6 +41,9 @@ path c:\cygwin\bin;%path%
 set CHERE_INVOKING=yes
 :: Workaround for "configure: Bad file descriptor"
 perl -i".bak" -pe "s/^test -n \".DJDIR\"/#$&/" configure
+:: Workaround for currently broken Python under Cygwin: don't use it for
+:: running gen_iface.py.
+touch include/wx/stc/stc.h
 bash -lc "g++ --version"
 bash -lc "LDFLAGS=-L/usr/lib/w32api ./configure --disable-optimise --disable-shared && make -j3"
 goto :eof