configure script adds -lc_r on FreeBSD instead of using -pthread (ITS#466)

[jb@cimlogic.com.au]

Full_Name: John Birrell
Version: Built from latest cvsup
OS: FreeBSD 4.0-CURRENT
URL: 
Submission from: (NULL) (139.130.51.31)


On FreeBSD, gcc will link against the libc_r (the threaded version of libc)
*instead* of libc iff you use the -pthread argument when linking.
If you use -lc_r instead, gcc will link the program against *both*
libc and libc_r. This will end in tears (core dump).

I'm not a fan of GNU configure so I have no (good) idea about how
to get the configure script to do the right thing.