Detect -lm in genlib

genlib's configure checks for -lmangle before -lm; as a result, the
-lm checks include -lmangle which fails because the required library
path isn't specified. To avoid this, we check for -lm before checking
for -lmangle.

Signed-off-by: Stephen Kitt <steve@sk2.org>
Signed-off-by: Liu Hao <lh_mouse@126.com>