m4/ax_prog_cc_for_build.m4: work around change in autoconf 2.70
m4/ax_prog_cc_for_build.m4: work around change in autoconf 2.70

AC_PROG_CC_FOR_BUILD uses the internal macros _AC_COMPILER_EXEEXT and
_AC_COMPILER_OBJEXT to set the variables BUILD_EXEEXT and BUILD_OBJEXT.
It does so by temporarily replacing EXEEXT by BUILD_EXEEXT,
ac_exeext by ac_build_exeext and ac_cv_exeext by ac_cv_build_exeext.
while executing _AC_COMPILER_EXEEXT (and similarly for OBJEXT).
Inside _AC_COMPILER_EXEEXT, EXEEXT is set (and used) in the two lines

    AC_SUBST([EXEEXT], [$ac_cv_exeext])dnl
    ac_exeext=$EXEEXT

This used to work in older versions of autoconf, as this is effectively
changed into

    AC_SUBST([BUILD_EXEEXT], [$ac_cv_build_exeext])dnl
    ac_build_exeext=$BUILD_EXEEXT

but since autoconf commit v2.69-18-g5fdd360b (AC_SUBST: don't underquote
the variable name, Wed Aug 15 21:54:53 2012 -0600), the EXEEXT
in the AC_SUBST call no longer gets replaced by BUILD_EXEEXT, resulting in

    AC_SUBST([EXEEXT], [$ac_cv_build_exeext])dnl
    ac_build_exeext=$BUILD_EXEEXT

This means not only that BUILD_EXEEXT is not getting set,
but also that the original EXEEXT is overwritten
by a possibly different value.  Furthermore, ac_build_exeext
is assigned an undefined value.

This is based on a patch by Jannick <thirdedition@gmx.net>.

Signed-off-by: Sven Verdoolaege <235c10dd23b819f81cdc9756a251746bc184cab6@gmail.com>

Meinersbur/isl,Meinersbur/isl,Meinersbur/isl,Meinersbur/isl