Log Message:
Fix "libtool: compile: unable to infer tagged configuration" by not
overwriting pkgsrc's setting of $(CC).

Also, make it build on amd64 netbsd (by cribbing two lines from the
existing openbsd support) and add that to the list of PLATFORMs the
package is ONLY_FOR.

XXX: Why is this limited to netbsd PLATFORMs when it supports at least
XXX: some other stuff internally?