AX_BUILD_CC_FOR_BUILD broken?

Hello

I'm not sure if I'm using the right terms here, please excuse me, as my 
knowledge of the internals of autoconf scripts doesn't reach too far 
beyond typing './autogen.sh' which invoke autoconf and convert the 
configure.am file to configure... :-)

I'm currently debugging a 3rd party project, which I'm compiling under 
Windows MinGW.  The project Makefile.am uses a few instances of 'EXEEXT' 
and 'BUILD_EXEEXT'.  However, the 'BUILD_EXEEXT' variable always turned 
out empty (where it should be '.exe').

I started debugging, and found out about the script responsible to 
define the 'BUILD_EXEEXT' variable, which is in autoconf-archive 
'ax_prog_cc_for_build.m4'.  I understood that this script basically just 
pushdef some 'important' definitions, disable cross-compilation (for the 
following test), re-call AC_PROG_CC, extract the 'BUILD_EXEEXT' from the 
results, and restore the 'important' definitions.

So, the idea is neat and simple, however, it does not seem to work as 
intended because when AC_PROG_CC_FOR_BUILD invoke the autoconf's 
AC_PROG_CC, the AC_PROG_CC macro (defined in c.m4) invoke 
_AC_COMPILER_EXEEXT (defined in lang.m4) with m4_expand_once.

So, the evaluation of executable extension occur only once for the whole 
configure script (and is not re-evaluated for the native build).

Regards,
Eric