`0'. Test and fix.
Reported by Assar Westerlund.
* Makefile.am (maintainer-check): New target.
* tests/Makefile.am (maintainer-check, maintainer-check-posix)
(maintainer-check-c++, expr): Likewise.
* configure.in (EXPR): Look for it.
* tests/atgeneral.m4 (AT_CHECK): Propagate $2's default value.
* tests/compile.at (Extensions): New test.
* aclang.m4 (_AC_COMPILER_EXEEXT_DEFAULT, _AC_COMPILER_EXEEXT_O):
Distinguish files with or without a dot.