THRIFT-2906 - linking with boost_unit_test_framework made more portable

Client: C++
Patch: Jon Parise

BOOST_TEST_LDADD Makefile variable added and should be used when linking
with boost_unit_test_framework

This closes #333
diff --git a/configure.ac b/configure.ac
index 6d55f8a..5b3d186 100755
--- a/configure.ac
+++ b/configure.ac
@@ -134,6 +134,8 @@
 if test "$with_cpp" = "yes";  then
   AX_BOOST_BASE([1.54.0])
   if test "x$succeeded" = "xyes" ; then
+    AC_SUBST([BOOST_LIB_DIR], [$(echo "$BOOST_LDFLAGS" | sed -e 's/^\-L//')])
+    AC_SUBST([BOOST_TEST_LDADD], [$(echo "$BOOST_LIB_DIR/libboost_unit_test_framework.a")])
     have_cpp="yes"
   fi