Perftest: enable inherit environment LDFLAGS

Sometimes, it needs to use environment to specify the library path
to be linked during development stage if the library doesn't exist
in system installed path. enable inherit environment LDFLAGS in configure
to support it.

Signed-off-by: Liu, Changcheng <changcheng.liu@aliyun.com>

configure.ac

@@ -310,6 +310,7 @@ if [test $HAVE_MLX5DV_LIB = yes] && [test $HAVE_MLX5DV = yes]; then
 fi
 
 CFLAGS="-g -Wall -D_GNU_SOURCE -O3 $CFLAGS"
+LDFLAGS="$LDFLAGS"
 LIBS=$LIBS" -lpthread"
 AC_SUBST([LIBUMAD])
 AC_SUBST([LIBMATH])