Trying to compile with --with-debug fails

Closes #17 . 

Commit 8074132 is a hack - comment out any access to SPA_VERSION_CRYPTO:
```
../../module/zfs/zio.c: In function ‘zio_decrypt’:
../../module/zfs/zio.c:404: error: ‘SPA_VERSION_CRYPTO’ undeclared (first use in this function)
../../module/zfs/zio.c:404: error: (Each undeclared identifier is reported only once
../../module/zfs/zio.c:404: error: for each function it appears in.)
../../module/zfs/zio.c: In function ‘zio_read_bp_init’:
../../module/zfs/zio.c:1065: error: ‘SPA_VERSION_CRYPTO’ undeclared (first use in this function)
../../module/zfs/zio.c: In function ‘zio_write_bp_init’:
../../module/zfs/zio.c:1251: error: ‘SPA_VERSION_CRYPTO’ undeclared (first use in this function)
```

Commit c276f2a is a hack - undefine AT_{UID,GID}. They will be defined properly later in the include chain:

```
/bin/sh ../../libtool --tag=CC --silent  --mode=compile gcc -DHAVE_CONFIG_H -include ../../zfs_config.h -I../../include -I../../lib/libspl/include  -D_GNU_SOURCE -D__EXTENSIONS__ -D_REENTRANT -D_POSIX_PTHREAD_SEMANTICS -D_FILE_OFFSET_BITS=64 -D_LARGEFILE64_SOURCE -DTEXT_DOMAIN=\"zfs-linux-user\"  -Wall -Wstrict-prototypes -fno-strict-aliasing  -DDEBUG -Werror -g -O2 -MT libzfs_fru.lo -MD -MP -MF .deps/libzfs_fru.Tpo -c -o libzfs_fru.lo `test -f '../../lib/libzfs/libzfs_fru.c' || echo './'`../../lib/libzfs/libzfs_fru.c
In file included from ../../include/sys/zio.h:32,
                 from ../../include/sys/zcrypt.h:29,
                 from ../../include/libzfs.h:41,
                 from ../../lib/libzfs/libzfs_fru.c:35:
../../include/sys/zfs_context.h:550:1: error: "AT_UID" redefined
In file included from /usr/include/link.h:25,
                 from ../../lib/libzfs/libzfs_fru.c:30:
/usr/include/elf.h:942:1: error: this is the location of the previous definition
In file included from ../../include/sys/zio.h:32,
                 from ../../include/sys/zcrypt.h:29,
                 from ../../include/libzfs.h:41,
                 from ../../lib/libzfs/libzfs_fru.c:35:
../../include/sys/zfs_context.h:551:1: error: "AT_GID" redefined
In file included from /usr/include/link.h:25,
                 from ../../lib/libzfs/libzfs_fru.c:30:
/usr/include/elf.h:944:1: error: this is the location of the previous definition
```

Commit 8d51e0a I'm unsure about - double check that I got the point correctly.