-libdir on Darwin

[Adriaan van Os]

In Autoconf, in lib/autoconf/general.m4, there is a code fragment that checks to "Be sure to have 
absolute directory names".

# Be sure to have absolute directory names.
for ac_var in   exec_prefix prefix bindir sbindir libexecdir datarootdir \
                datadir sysconfdir sharedstatedir localstatedir includedir \
                oldincludedir docdir infodir htmldir dvidir pdfdir psdir \
                libdir localedir mandir
do
  eval ac_val=\$$ac_var
  case $ac_val in
    [[\\/$]]* | ?:[[\\/]]* )  continue;;
    NONE | '' ) case $ac_var in *prefix ) continue;; esac;;
  esac
  AC_MSG_ERROR([expected an absolute directory name for --$ac_var: $ac_val])
done

Among others, libdir is checked to have an absolute directory name. However, on Darwin the 
-install_name of a dynamic library (or Darwin bundle file 
<http://developer.apple.com/documentation/Darwin/Reference/ManPages/man1/ld.1.html>) can start with 
the '@executable_path' macro, see 
<http://developer.apple.com/documentation/DeveloperTools/Conceptual/MachOTopics/Articles/loading_code.html>. 
Therefore, the check for an absolute pathname for libdir in the above code fragment is wrong.

I suggest to
* remove libdir from the code fragment, or
* skip libdir there for Darwin targets only, or
* extend the test to allow libdir (or any path) to start with '@executable_path' or more generic 
with '@'.

On Mac OS X, in an application package, the -install_name of a dynamic library is typically 
'@executable_path/../lib' or '@executable_path/../Frameworks/.....'.

GNU libtool has the same issue, but I will report that separately.

Sincerely,

Adriaan van Os
address@hidden
<http://www.microbizz.nl/gpc.html>