havelib improvements

[Bruno Haible]

Hi,

In many installations, clang has strange/odd/buggy library search paths. In
particular, on the Ubuntu binaries from http://releases.llvm.org/download.html
search in /lib64/ although this directory contains no libraries (only the 
ld.so).
This confuses the AC_LIB_PREPARE_MULTILIB macro: it expects 64-bit libraries
in $PREFIX/lib64 (this hack was implemented for the sake of openSUSE systems).
But in Ubuntu, 64-bit binaries are in $PREFIX/lib.

I'm adding two patches:

1) The ability for the user to override the results of AC_LIB_PREPARE_MULTILIB.
   With this patch, I can run the configuration with
      acl_cv_libdirstems=lib,lib64 CC=clang ./configure 
--with-libsigsegv-prefix=...
   and it (i.e. gl_LIBSIGSEGV) will find the installed 64-bit binaries of
   libsigsegv, whereas before with just
      CC=clang ./configure --with-libsigsegv-prefix=...
   it did not find them.

2) Prefer to ask the system compiler (/usr/bin/gcc) about the characteristics of
   the system, rather than $CC.

0001-havelib-Support-overriding-the-result-of-AC_LIB_PREP.patch
Description: Text Data

0002-havelib-Prefer-the-search-path-of-usr-bin-gcc-over-t.patch
Description: Text Data