the same gcc binary emits warnings on one machine but not on another

[john smith]

We use the same cross-compiler on two Linux machines: machine A and
machine B.  It produces some warnings on machine A but not on machine
B when used to compile the same file and with the same set of options
(specifically we use -Wall and -Wextra).  We are absolutely sure that
it's the same compiler - it's in the same version and has the same
md5sum on both machines.  However, these 2 Linux machines differ - A
is x32 Ubuntu, B is x64 bit Slackware.  I checked all environment
variables on both machines and didn't find anything suspicious - no
CFLAGS, CC or something related to compilation at all.  The compiler
in question behaves the same way when invoked from Makefile and when
invoked directly from the command line on both machines.

Is this possible that due to some local differences in
libc/libgcc/whatnot GCC emits warnings on machine A but not on machine
B?  What else can I check to learn what's the culprit here?
-- 
<wempwer@gmail.com>