The '-Wshadow' flag  is broken in gcc and prevents valid code to be compiled when '-Werror' flag is used. 

This is the case when enable_debug=full.
For example, 'xfce4-settings' cannot be compiled with enable_debug=full.

xdt-features.m4 needs a patch to remove '-Wshadow' when enable_debug='full'. It can be kept with enable_debug='yes'

http://git.xfce.org/xfce/xfce4-dev-tools/tree/m4macros/xdt-features.m4

Arguments:
https://lkml.org/lkml/2006/11/28/239
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=57709