Remove AM_PROG_AR.
With AM_PROG_AR may report "error: possibly underfined macro: AM_PROG_AR" (mingw, autoreconf-2.68).
Without AM_PROG_AR may report warning "archiver requires 'AM_PROG_AR' in 'configure.ac'" (cygwin, automake-1.12).
They do not affect the tests and install


git-svn-id: http://protobuf.googlecode.com/svn@545 630680e5-0e50-0410-840e-4b1c322b438d