Fix build with --as-needed

https://bugs.gentoo.org/268094

--- a/configure.in
+++ b/configure.in
@@ -289,7 +289,7 @@
 if test "$enable_pthreads" = yes; then
     echo "Using PTHREADS"
     CFLAGS="$CFLAGS -DUSE_PTHREADS"
-    LDFLAGS="$LDFLAGS -lpthread"
+    LIBS="$LIBS -lpthread"
 elif test "$enable_pthreads" = no; then
     echo "Not using pthreads"
 else