configure: work around a bug in AX_PROG_CC_FOR_BUILD...

... when building on Windows.
From bug 780270, comment 18:
I'm still having issue with Windows MinGW, but I have traced the issue
with the autoconf itself, and the autoconf-archive script
"ax_prog_cc_for_build.m4". I have written to the autoconf-archive
mailing list.

It seem that this script never worked as intended since a long time
because the way it works, it pushdef a few elements, then it disable
cross-compiling (for the following test), and invoke AC_PROG_CC (which
in turn invoke the code that find and set the exe extention). Then it
grab the BUILD_EXEEXT from that. This is neat and simple, but the issue
is that the autoconf AC_PROG_CC macro only invoke the code that is
responsible for finding the exe (and obj) extensions once (with
m4_expand_once). So, the end-result is that in the resulting configure
script, EXEEXT is properly evaluated, but when comes the time to
evaluate BUILD_EXEEXT, no test is performed to actually find the exe
(and obj) extension, even if the cross-compilation option changed (which
is the case for the duration of this test).

So, BUILD_EXEEXT will always end up blank (defined, but blank).

configure.ac

@@ -233,6 +233,11 @@ AS_IF([test "x$0" != "x./configure"], [
 # Determine a C compiler to use
 AC_PROG_CC
 AX_PROG_CC_FOR_BUILD
+# Works around a bug in AX_PROG_CC_FOR_BUILD for build with MinGW on
+# Windows. See bug 780270, comment 18.
+case "$build_os" in
+  cygwin*|mingw32*|mingw64*)	BUILD_EXEEXT=.exe ;;
+esac
 AM_PROG_CC_C_O
 
 # Determine a C++ compiler to use