Parameterize pthread and iconv flags

[Harald Eilertsen]

Hi,

As suggested by @magicus, I have split out a couple of trivial patches
to move the compiler and linker flags for pthread and iconv support to
be pupulated by the configure scripts.

As any PR needs to have an assigned bug ID, I have tried to submit
enhancement requests for these through bugreport.java.com. Not sure how
long time that usually takes before it get's published (or I hear
anything back,) but if anyone could have a look, the internal ID's are
9078187 (pthread) and 9078196 (iconv).

For now, both of these will probably seem entirely unnecessary, but they
are needed for the BSD port, and it was suggested to split them out and
merge them to prepare for the port.

Take care!
Harald