hdf5_ep

[svn-r7150] Purpose:
    Bug fix

Description:
    "prefix" variable was always being set, instead of only being set if the
    user hadn't set it yet.

Solution:
    Changed "prefix=<foo>" to "prefix?=<foo>".

Platforms tested:
    FreeBSD 4.8 (sleipnir)
    Checked in gmake manual to make certain syntax is the same (it is).