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
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