Only use setenv() on Solaris if it is available.

This is a followup to 07ad9e0e19891ec129e1a78e40a66ca19b51302d.
util.c:Perl_my_setenv() unconditionally used setenv() if #defined(__sun).
Solaris 8 had neither setenv() nor unsetenv(), so it failed.  Configure
does not currently check for setenv(), but the check for unsetenv()
is sufficient here since Solaris 9 and later have both.