if --with_boost is specified, search *only* the specified directory hierarchy for libraries.

Arguably, if --with_boost is specified, the user wants to use only
that boost installation, and no other.  This changeset implements this
behavior.

The original code in _BOOST_FIND_LIBS prepended the --with_boost path
to the default list of paths, resulting in all of the paths being
searched if the library wasn't found in that specified by
--with_boost.  Because the search for a library begins with the most
specialized version:

    boost_$boost_lib_$boost_tag_$boost_mt_$boost_rtopt_$boost_ver_
    boost_$boost_lib_$boost_tag_$boost_rtopt_$boost_ver_
    boost_$boost_lib_$boost_tag_$boost_mt_$boost_ver_
    boost_$boost_lib_$boost_tag_$boost_ver_

if the user's preferred boost installation has a less specialized
version of a library than one provided in the default path, the latter
will be chosen instead.

This may lead to a mismatch between the headers discovered in the
--with_boost hierarchy and the library discovered in the default path,
if they are for different versions of boost, leading to link problems
if the latter is for an earlier version .

Additionally, this will silently invalidate a request for a specific
version of boost in BOOST_REQUIRE, as it will be fulfilled by the
headers, but not by the library.

The new code does the following:

* It will only search the --with_boost hierarchy if that is provided;

* It will (following the suggestion of issue #49 by mateidavid) use the
  absolute path to the library so that the linker's default
  library paths are not searched if the library is not found in the
  --with_boost hierarchy

* It adds a second path "$with_boost/lib" to the search path, as that
  seems to be a standard place for libraries

N.B. There is still a potential for mishap if --with_boost is *not*
specified, namely that if there exist multiple boost installations and
some do not have all of the libraries, it is possible to end up with
libraries selected from multiple installations.

Author: djerius

build-aux/boost.m4

@@ -47,6 +47,16 @@ m4_define([_BOOST_SERIAL], [m4_translit([
 
 m4_pattern_forbid([^_?(BOOST|Boost)_])
 
+# _BOOST_SHELL_FUNCTIONS
+# --------------------------------------------------------
+#
+# Useful shell functions. Call
+#   AC_REQUIRE([_BOOST_SHELL_FUNCTIONS])
+# before using
+AC_DEFUN([_BOOST_SHELL_FUNCTIONS],
+[_boost_join_path() { _boost_join_path_save_IFS=$IFS; IFS=@ ; echo "$[]*" ; IFS=$_boost_join_path_save_IFS ; } ]
+)
+
 
 # _BOOST_SED_CPP(SED-PROGRAM, PROGRAM,
 #                [ACTION-IF-FOUND], [ACTION-IF-NOT-FOUND])
@@ -370,7 +380,8 @@ AC_DEFUN([BOOST_FIND_LIB],
 # to enforce -mt (for instance on MacOSX, libboost_thread.dylib
 # doesn't exist but there's -obviously- libboost_thread-mt.dylib).
 AC_DEFUN([_BOOST_FIND_LIBS],
-[Boost_lib=no
+[AC_REQUIRE([_BOOST_SHELL_FUNCTIONS])
+Boost_lib=no
   case "$3" in #(
     (mt | mt-) boost_mt=-mt; boost_rtopt=;; #(
     (mt* | mt-*) boost_mt=-mt; boost_rtopt=`expr "X$3" : 'Xmt-*\(.*\)'`;; #(
@@ -435,27 +446,37 @@ for boost_rtopt_ in $boost_rtopt '' -d; do
     case $boost_failed_libs in #(
       (*@$boost_lib@*) continue;;
     esac
-    # If with_boost is empty, we'll search in /lib first, which is not quite
-    # right so instead we'll try to a location based on where the headers are.
-    boost_tmp_lib=$with_boost
-    test x"$with_boost" = x && boost_tmp_lib=${boost_cv_inc_path%/include}
-    for boost_ldpath in "$boost_tmp_lib/lib" '' \
-             /opt/local/lib* /usr/local/lib* /opt/lib* /usr/lib* \
-             "$with_boost" C:/Boost/lib /lib*
+    # If with_boost is specified; search *only* within that hierarchy,
+    # otherwise search the "standard" places, starting with a location
+    # based upon where the headers are.
+    AS_IF( [ test x"$with_boost" = x ],
+	  [ boost_ldpaths=`_boost_join_path ${boost_cv_inc_path%/include} ''  \
+	                  /opt/local/lib* /usr/local/lib* /opt/lib* /usr/lib* \
+		          C:/Boost/lib /lib*` ],
+          [ boost_ldpaths=`_boost_join_path "$with_boost" "$with_boost/lib"` ]
+    )
+
+    save_IFS=$IFS
+    IFS=@
+    for boost_ldpath in `echo "$boost_ldpaths"`
     do
+      IFS=$save_IFS
       # Don't waste time with directories that don't exist.
       if test x"$boost_ldpath" != x && test ! -e "$boost_ldpath"; then
         continue
       fi
       boost_save_LDFLAGS=$LDFLAGS
+      # use an absolute path to the requested library to avoid finding it
+      # in the linker's default search path
       # Are we looking for a static library?
       case $boost_ldpath:$boost_rtopt_ in #(
         (*?*:*s*) # Yes (Non empty boost_ldpath + s in rt opt)
-          Boost_lib_LIBS="$boost_ldpath/lib$boost_lib.$libext"
-          test -e "$Boost_lib_LIBS" || continue;; #(
-        (*) # No: use -lboost_foo to find the shared library.
-          Boost_lib_LIBS="-l$boost_lib";;
+          Boost_lib_LIBS="$boost_ldpath/lib$boost_lib.$libext" ;;
+        (*) # No:
+          Boost_lib_LIBS="$boost_ldpath/lib$boost_lib$shrext_cmds" ;;
       esac
+      # Don't waste time with libraries that don't exist
+      test -e "$Boost_lib_LIBS" || continue
       boost_save_LIBS=$LIBS
       LIBS="$Boost_lib_LIBS $LIBS"
       test x"$boost_ldpath" != x && LDFLAGS="$LDFLAGS -L$boost_ldpath"