title: Autotools: top_srcdir and abs_top_srcdir are empty
body: <p>i'm currently trying to include a libtool library from another project that lies beside my current project. I can check the library in configure using </p>
  
  <pre><code>LDFLAGS="$LDFLAGS -L$top_srcdir../otherproject/libotherproject/.libs/"
  AC_CHECK_LIB([otherproject],[init],[],[AC_MSG_ERROR([No otherproject libary found.])])
  </code></pre>
  
  <p>in my configure.ac. Everything is fine so far. But if i build the project using make, i get an error from libtool while it is in currentproject/sources/: </p>
  
  <pre><code>../libtool: line 5986: cd: ../otherproject/libotherproject/.libs/: No such file or directory
  libtool: link: cannot determine absolute directory name of `../otherproject/libotherproject/.libs/'
  </code></pre>
  
  <p>Which is logical, because it should be something like <code>../../otherproject/libotherproject/.libs/</code>. I tried to debug that and found that if is use </p>
  
  <pre><code>AC_MSG_NOTICE([Top src dir is: $top_srcdir])
  </code></pre>
  
  <p>in configure.ac, the config script tells me</p>
  
  <pre><code>configure: Top src dir is: 
  </code></pre>
  
  <p>only. Same behaviour for $abs_top_srcdir. $srcdir is "." always. I digged a bit in the documentation and found that $builddir should always be ".", but is also empty in my case. Is this a bug? Maybe i forgot to call a <code>AC_init_anything</code> ? Thanks for your help! </p>