Fix `rand_r` detection in autotools.
Fix `rand_r` detection in autotools.

mongodb/libbson,rcsanchez97/libbson,mongodb/libbson,ajdavis/libbson,ajdavis/libbson,rcsanchez97/libbson,rcsanchez97/libbson,mapr/libbson,ajdavis/libbson,mongodb/libbson,mapr/libbson,mapr/libbson