Bug in AX_BOOST_* macros

Hi all!

I just found an annoying, but easy to fix bug in all of the AX_BOOST_*
macros.

Steps to reproduce
------------------

Run checks for more than one boost library, e.g.:

AX_BOOST_BASE([1.46.0])
AX_BOOST_PROGRAM_OPTIONS
AX_BOOST_THREAD

Make sure that the library file for boost program options is found,
but the library file for boost thread is missing.


Expected behaviour
------------------

The second macro errors out.


Actual behaviour
----------------

The second macro succeeds.


Bug description
---------------

AX_BOOST_PROGRAM_OPTIONS and AX_BOOST_THREAD (and all the other macros
I looked at briefly) all use the same variable $ax_lib to check for
the name of the library file to pass to the linker.

Running two macros after each other means that if the first macro
found a library file, the $ax_lib variable is already set before the
second macro is run. As a result, the second macro does not detect the
missing library file - instead, it assumes that the library it found
is the one from the first macro.


Proposed Fix
------------

Simply set $ax_lib to an empty value before the checks for the library
file are performed.

I've attached an example patch for AX_BOOST_THREAD.


Hope that helps,
  Jens

-- 
1.21 Jiggabytes of memory should be enough for anybody.

diff --git a/m4/ax_boost_thread.m4 b/m4/ax_boost_thread.m4
index 79e12cd..d1fa817 100644
--- a/m4/ax_boost_thread.m4
+++ b/m4/ax_boost_thread.m4
@@ -103,6 +103,7 @@ AC_DEFUN([AX_BOOST_THREAD],
                           break;
                           ;;
                         esac
+            ax_lib=
             if test "x$ax_boost_user_thread_lib" = "x"; then
                 for libextension in `ls -r $BOOSTLIBDIR/libboost_thread* 2>/dev/null | sed 's,.*/lib,,' | sed 's,\..*,,'`; do
                      ax_lib=${libextension}

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