SCons implements special handling for a number of platforms, notably
older UNIX variants. Unfortunately it does not carry this handling
through to the rest of its infrastructure, and so these platforms are
missing fundamental features such as shared library support.
Rather than treating it as a special case and adding "or sunos" to
every single platform check, leave it to be detected as the default
'posix' platform. The only loss is that the sunos platform file is
no longer loaded, however there is little in there of value to us.
This has the added benefit of fixing the assumption SCons makes that
all SunOS platforms must use the Sun Studio tools, and ensures that we
are able to use GCC too.
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