title: MacOS Mojave: ./configure script fails when checking the archiver (ar) interface
body: <p>I am running the <code>./configure</code> command before building the OpenFST package, which uses autoconf.
  The script fails when checking for the <code>ar</code> interface:</p>
  
  <pre><code>$ ./configure
  checking for a BSD-compatible install... /usr/bin/install -c
  checking whether build environment is sane... yes
  checking for a thread-safe mkdir -p... ./install-sh -c -d
  checking for gawk... gawk
  checking whether make sets $(MAKE)... yes
  checking whether make supports nested variables... yes
  checking for style of include used by make... GNU
  checking for gcc... gcc
  checking whether the C compiler works... yes
  checking for C compiler default output file name... a.out
  checking for suffix of executables...
  checking whether we are cross compiling... no
  checking for suffix of object files... o
  checking whether we are using the GNU C compiler... yes
  checking whether gcc accepts -g... yes
  checking for gcc option to accept ISO C89... none needed
  checking whether gcc understands -c and -o together... yes
  checking dependency style of gcc... gcc3
  checking for ar... ar
  checking the archiver (ar) interface... unknown
  configure: error: could not determine ar interface
  </code></pre>
  
  <p>This seems related to <a href="https://stackoverflow.com/questions/47074736/autoconf-uses-wrong-ar-on-os-x">this question</a> but I can't understand how to make the script use the correct MacOS file so that configuration would work.</p>