configure.in: Fix detection of Boost libraries. Reported by <plug.gulp@gmail.com>.

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@12507 a5019735-40e9-0310-863c-91ae7b9d1cf9
diff --git a/configure.in b/configure.in
index b6eb371..177e58a 100644
--- a/configure.in
+++ b/configure.in
@@ -1868,7 +1868,9 @@
 
 AC_LANG(C++)
 safe_CXXFLAGS=$CXXFLAGS
-CXXFLAGS="-lboost_thread-mt $mflag_primary"
+CXXFLAGS="$mflag_primary"
+safe_LIBS="$LIBS"
+LIBS="-lboost_thread-mt $LIBS"
 
 AC_LINK_IFELSE([AC_LANG_SOURCE([
 #include <boost/thread.hpp>
@@ -1890,6 +1892,7 @@
 AC_MSG_RESULT([no])
 ])
 
+LIBS="$safe_LIBS"
 CXXFLAGS=$safe_CXXFLAGS
 AC_LANG(C)