vchoi_fork

[svn-r4765] Purpose:
    Bug fix.
Description:
    FreeBSD would fail the threadsafe feature if static-exec is not on.
Solution:
    Force enable-static-exec on if enable-threadsafe is on.
    Also moved the chunk of FreeBSD specific code for Pthread setup
    from configure.in to config/freebsd.
    Also changed enable-threadsafe to check on linking pthread program.
    That takes care of platforms (e.g. freebsd) that has pthread
    support builtin the default C library.  Now one can just use
    "enable-threadsafe" if the compiler has pthread support by default.
Platforms tested:
    eirene, Sleipnir