New feature in AC_PROG_CHECK

[Mohammad Akhlaghi]

Hi Autoconf maintainers,

In the `configure.ac' of GNU Astronomy Utilities (Gnuastro), I need to 
check if `libtool' is present on the system or not (independent of the 
libtool that is shipped with Gnuastro and is used for its internal 
building) [P.S1]. Therefore, I use

AC_CHECK_PROG(has_libtool, libtool, [yes], [no])

Recently a user reported that while they didn't have libtool on their 
system, this test had passed. From `config.log', I could see that it had 
found (`.//libtool'). Upon further investigation, I found out that it 
was because his PATH included the current directory and for some reason, 
it was finding Gnuastro's internally built Libtool [P.S2].

So I updated the command above to this:

AC_CHECK_PROG(has_libtool, libtool, [yes], [no], , .//libtool)

But today, another user reported a similar problem and when I looked at 
their `config.log' I found out that on that system, it had found 
`./libtool'! So I changed the command to this:

AC_CHECK_PROG(has_libtool, libtool, [yes], [no], , .//libtool ./libtool)

But looking at the configure script, I noticed that the last argument to 
AC_CHECK_PROG must only be a single file, not multiple. The only 
solution that I have come up with so far is call it two times like below.

AC_CHECK_PROG(has_libtool, libtool, [yes], [no], , ./libtool)
AC_CHECK_PROG(has_libtool, libtool, [yes], [no], , .//libtool)

I wanted to consult you on the best way forward.

I also wanted to see if it would be possible to allow the last argument 
to AC_CHECK_PROG to accept multiple file names, not just one? In this 
way, a form like my first try would work and it would also be much more 
efficient.

Thank you very much,
Mohammad



P.S1. Libtool is necessary for one of Gnuastro's programs to work: 
BuildProgram (see link below). After installation, it uses the user's 
system's libtool to fix complications with include file and library 
directories. The description is given here:

https://www.gnu.org/software/gnuastro/manual/html_node/BuildProgram.html



P.S2. Gnuastro does come with its own implementation of libtool (built 
from `libtool.m4). But it is strange that the configure script has found 
libtool in the current directory. Because at least on my system, the 
configure script builds `libtool' from `libtool.m4' after all the checks 
not before it. But any way, this is besides the main point here.