failing configure => dump config.log in the log

[Akim Demaille]

Hi!

For some reason, the configure step of Vaucanson fails on
SnowLeopard; this log:

> https://build.macports.org/builders/buildports-snowleopard-x86_64/builds/22196/steps/compile/logs/stdio

ends with:

> checking boost/regex.hpp usability... no
> checking boost/regex.hpp presence... yes
> configure: WARNING: boost/regex.hpp: present but cannot be compiled
> configure: WARNING: boost/regex.hpp:     check for missing prerequisite headers?
> configure: WARNING: boost/regex.hpp: see the Autoconf documentation
> configure: WARNING: boost/regex.hpp:     section "Present But Cannot Be Compiled"
> configure: WARNING: boost/regex.hpp: proceeding with the compiler's result
> configure: WARNING:     ## ------------------------------------------- ##
> configure: WARNING:     ## Report this to vaucanson-bugs at lrde.epita.fr ##
> configure: WARNING:     ## ------------------------------------------- ##
> checking for boost/regex.hpp... no
> configure: error: cannot find boost/regex.hpp
> Command failed:  cd "/opt/local/var/macports/build/_opt_mports_dports_devel_vaucanson/vaucanson/work/vaucanson-2b.0" && ./configure --prefix=/opt/local 
> Exit code: 1
> 
> Error: Failed to configure vaucanson, consult /opt/local/var/macports/build/_opt_mports_dports_devel_vaucanson/vaucanson/work/vaucanson-2b.0/config.log
> Error: org.macports.configure for port vaucanson returned: configure failure: command execution failed
> DEBUG: Error code: NONE
> DEBUG: Backtrace: configure failure: command execution failed
>    while executing
> "$procedure $targetname"

Unfortunately such a message without any additional details
is almost useless: what really matters in the case of an
Autoconf's configure run is the config.log file.

So I would like to suggest that if configure fails, then
find should be run to find all the possible config.log files,
and dumps them.  This should address two concerns of Ryan's:
if there is no config.log (because e.g., configure is not
from Autoconf), everything is fine, and if there are several
configure (e.g., when using AC_CONFIG_SUBDIRS for subpackages),
all the config.logs files are dump.

Thanks!