* aclang.m4 (_AC_PROG_CC_G): Use the _AC_COMPILE_IFELSE macro

* aclang.m4 (_AC_PROG_CC_G): Use the _AC_COMPILE_IFELSE macro
instead of reinventing the wheel. This also takes care of a bug
where the "-g" option was accepted if the compiler silently exits
with status unequal to 0.
(_AC_PROG_CXX_G): Likewise.
(AC_PROG_CC): As _AC_PROG_CC_G depends on $ac_objext and
$ac_exeext, move the expansion of _AC_OBJEXT and _AC_EXEEXT in
front of _AC_PROG_CC_G.
(AC_PROG_CXX, AC_PROG_F77): Likewise.