pyside option to enable/disable pyi files generation

because pyi file generation may fail on some platforms

(see https://bugreports.qt.io/browse/PYSIDE-1193, or on conda/windows)

I propose to add an option to enable/disable pyi files generation to opt-out on the problematic platforms

if (PYSIDE_IS_CROSS_BUILD) 
  option (PYSIDE_GENERATE_PYI "generate pyi files" OFF) 
else () 
  option (PYSIDE_GENERATE_PYI "generate pyi files" ON) 
endif () 
if(PYSIDE_GENERATE_PYI) 
     ...

here is a proposition of patch, which keeps the current default

cc ctismer