pngcp: remove GNU setjmp warning workround

Prior versons of the GCC warned about the 'dest' parameter of
contrib/tools/pngcp.c not being volatile, which isn't necessary because
it isn't modified.  This removes the GCC specific fixup.

The function which calls setjmp, cppng() also relied on undefined
behavior because it assigned the result of setjmp() to a variable; this
is not one of the four uses of setjmp permitted by ANSI-C.  This passes
the result previously returned by longjmp via (struct display).  It's
very very unlikely that any compiler could have got the code wrong but
it is technically undefined.
1 file changed