Fix runtime error on OpenBSD 5.3

Hi,

This pull request fixes the following error on OpenBSD 5.3:

    /usr/local/lib/erlang/erts-5.10.3/bin/beam:/tmp/yaws/priv/lib/setuid_drv.so: undefined symbol '__guard_local'
    =ERROR REPORT==== 20-May-2013::03:37:28 ===
    Failed to load setuid_drv (from "/tmp/yaws/priv/lib") : "Cannot load specified object".

It follows advice from http://erlang.org/pipermail/erlang-questions/2013-May/073837.html and replaces the use of 'ld' with 'gcc' (through '$CC') in LD_SHARED.

I tested the patch on both OpenBSD 5.3 and FreeBSD 9 and I _think_ it should be safe for the other BSDs (although I didn't test them all).

Thanks,
Francis