Require less current autotools so we build on MacOS.

I didn't review things particularly carefully, but I don't see
any features the build scripts are depending on from newer
autotools releases except for AM_SILENT_RULES which can be
handled with m4_ifdef.

In any case, this seems to work fine with the autoconf 2.61
and automake 1.10 supplied by XCode 4.5.2.