byteswap side-effect defect report from Coverity

[Paul Eggert]

Bruno and I get defect reports from Coverity Scan for Gnulib. The most 
recent one has this new complaint:

> *** CID 1588680:  Incorrect expression  (ASSERT_SIDE_EFFECT)
> /gltests/test-byteswap.c: 43 in test_bswap_eval_once()
> 37     
> 38     /* Test that the bswap functions evaluate their arguments once.  */
> 39     static void
> 40     test_bswap_eval_once (void)
> 41     {
> 42       uint_least16_t value_1 = 0;
> > > > >>>     CID 1588680:  Incorrect expression  (ASSERT_SIDE_EFFECT)
> > > > >>>     Argument "value_1++" of ASSERT() has a side effect.  The containing 
> > > > function might work differently in a non-debug build.
> 43       ASSERT (bswap_16 (value_1++) == 0);

I checked the glibc sources, and long ago glibc indeed had a bug in this 
area: bswap_16 and related function-like macros evaluated their 
arguments more than once. This bug was fixed in the following glibc 
commit dated Sat Aug 8 20:02:34 1998 +0000, and the fix appears in glibc 
2.1 (1999).

https://sourceware.org/git/?p=glibc.git;a=commitdiff;h=7ce241a03e2c0b49482d9d05c8ddb765e89a01d9


Gnulib does not support glibc 2.1.x and older, so this should not be a 
problem when porting to glibc. However, I worried that other platforms 
might have the bug, until I noticed that m4/byteswap.m4 already 
inadvertently tests for it. I installed the attached patch to document 
the test.

We can ignore this Coverity defect report as a false positive, just as 
we ignore many other defect reports.

0001-m4-byteswap.m4-Add-comment-about-broken-C-libraries.patch
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