Warnings/errors related to possibly clobbered variables

[Dmitry Antipov]

With:

configure --prefix=/not/exists --enable-checking 
--enable-check-lisp-object-type --enable-gcc-warnings --with-x-toolkit=lucid

I got:

../../emacs/src/keyboard.c: In function ‘read_char’:
../../emacs/src/keyboard.c:2376:15: warning: variable ‘c’ might be clobbered by 
‘longjmp’ or ‘vfork’ [-Wclobbered]
   Lisp_Object c;
               ^
But with:

configure --prefix=/not/exists --enable-checking 
--enable-check-lisp-object-type --enable-gcc-warnings --with-x-toolkit=no

the same warning is treated as fatal error:

../../emacs/src/keyboard.c: In function ‘read_char’:
../../emacs/src/keyboard.c:2376:15: error: variable ‘c’ might be clobbered by 
‘longjmp’ or ‘vfork’ [-Werror=clobbered]
   Lisp_Object c;
               ^
cc1: all warnings being treated as errors

Why this is so?

Dmitry