Enable PostreSQL 9.4 Native UUID Support

This enables the native UUID support in Postgres 9.4 Beta2 on Snow Leopard and later.

So far this was only tested on Yosemite DP6 in the following combinations:
* Postgres 9.3.5 + uuid-ossp 1.6.2
* Postgres 9.4beta2 + native UUID support from MacOS X SDK

I've left the ability to build against uuid-ossp explicitly, although it's broken with 9.4 on Yosemite due to conflicts between the native and ossp uuid.h in the configure scripts, but maybe it is useful for pre Snow Leopard users.

UUID Test on 9.4.5 / uuid-ossp:
```
select uuid_generate_v1(), uuid_generate_v1mc(), uuid_generate_v3(uuid_ns_url(), 'http://www.postgresql.org'), uuid_generate_v4(), uuid_generate_v5(uuid_ns_url(), 'http://www.postgresql.org'), uuid_nil(), uuid_ns_dns(), uuid_ns_url(), uuid_ns_oid(), uuid_ns_x500();
-[ RECORD 1 ]------+-------------------------------------
uuid_generate_v1   | 21c1d7b4-2995-11e4-80b5-mac-removed!
uuid_generate_v1mc | 21c1d854-2995-11e4-80b6-43c63209cc19
uuid_generate_v3   | cf16fe52-3365-3a1f-8572-288d8d2aaa46
uuid_generate_v4   | 20e2de4c-0c7d-422e-b229-551b22ec3b75
uuid_generate_v5   | e1ee1ad4-cd4e-5889-962a-4f605a68d94e
uuid_nil           | 00000000-0000-0000-0000-000000000000
uuid_ns_dns        | 6ba7b810-9dad-11d1-80b4-00c04fd430c8
uuid_ns_url        | 6ba7b811-9dad-11d1-80b4-00c04fd430c8
uuid_ns_oid        | 6ba7b812-9dad-11d1-80b4-00c04fd430c8
uuid_ns_x500       | 6ba7b814-9dad-11d1-80b4-00c04fd430c8
```

And using native UUID support on 9.4beta2:
```
select uuid_generate_v1(), uuid_generate_v1mc(), uuid_generate_v3(uuid_ns_url(), 'http://www.postgresql.org'), uuid_generate_v4(), uuid_generate_v5(uuid_ns_url(), 'http://www.postgresql.org'), uuid_nil(), uuid_ns_dns(), uuid_ns_url(), uuid_ns_oid(), uuid_ns_x500();
-[ RECORD 1 ]------+-------------------------------------
 uuid_generate_v1   | edc5fccc-2996-11e4-a9dd-mac-removed!
 uuid_generate_v1mc | edc5fd12-2996-11e4-a9dd-e792bb0409e4
 uuid_generate_v3   | cf16fe52-3365-3a1f-8572-288d8d2aaa46
 uuid_generate_v4   | a02d123a-e4e6-411e-a88e-28cbc983f205
 uuid_generate_v5   | e1ee1ad4-cd4e-5889-962a-4f605a68d94e
 uuid_nil           | 00000000-0000-0000-0000-000000000000
 uuid_ns_dns        | 6ba7b810-9dad-11d1-80b4-00c04fd430c8
 uuid_ns_url        | 6ba7b811-9dad-11d1-80b4-00c04fd430c8
 uuid_ns_oid        | 6ba7b812-9dad-11d1-80b4-00c04fd430c8
 uuid_ns_x500       | 6ba7b814-9dad-11d1-80b4-00c04fd430c8
```
Note that only v3 and v5 are non-random and can be compared, but the results are all valid UUIDs.
Please break this out into an if/else, this is too hard to read.

Also, we don't want to introduce new usages of `build.stable?` (or `build.devel?` or `build.head?`) in the DSL, so this needs to be expressed using `stable`/`devel`/`head` blocks instead (even if it means a small amount of duplication).