* Added new cross-platform boost/build-boost script.
* Added new LINK_MODE var (static|shared) that is
used by Boost, PolarSSL, and OpenSSL (Linux) builds.
* More DEP_DIR flexibility:
(a) DEP_DIR can be defined prior to call of build-all scripts.
(b) On Linux, vars-linux script will not override DEP_DIR.
* build-all can be run from any directory, will still use
~/src/mac to write products.
* to make sure that scripts don't trigger a false error exit
by their caller, add "exit 0" to last line of all scripts
called by build-all.
