Re: [PATCH] Don’t use AC_EGREP_CPP if affected by CFLAGS

[Paul Eggert]

On 11/06/2017 02:08 AM, Bruno Haible wrote:
> This configuration issue comes up occasionally
It's more than "occasionally", I'm afraid. I just did a Google search 
for 'configure CFLAGS "-m32"' and in the first ten hits, more people got 
it wrong than right. When the error rate is this high, it's appropriate 
to change the code to make errors less likely.

>    1) AC_COMPILE_IFELSE is slower than AC_EGREP_CPP, and there is common
>       consensus (discussion on gnu-prog-discuss in October 2016) that configure
>       scripts are already slower than it should be.

Yes, and that's a real problem. However, these changes won't hurt 
performance much, as they're only a small number of tests.

>    2) There are many more uses of AC_EGREP_CPP outside of gnulib.

Yes, and there are many other uses of AC_EGREP_CPP even inside Gnulib. 
The patch does not eliminate all uses of AC_EGREP_CPP, only the uses 
where it's common for builders to use idioms like "./configure 
CFLAGS='-m32'" where CFLAGS unexpectedly affects the behavior that the 
AC_EGREP_CPP code is testing for. Many builders don't know that -m32 
affects the preprocessor, and we can't really expect them to read some 
obscure corner of the Gnulib manual to find this stuff out.