Make gomdb statically linked

Currently it is impossible to `go get github.com/szferi/gomdb` without having `lmdb.h` and `lmdb` library installed:

```
root@7fc2488c2bcc:/bugaga# go get github.com/szferi/gomdb
# github.com/szferi/gomdb
/go/src/github.com/szferi/gomdb/val.go:9:18: error: lmdb.h: No such file or directory

root@7fc2488c2bcc:/bugaga# cp /go/src/github.com/szferi/gomdb/lmdb.h /usr/include/
root@7fc2488c2bcc:/bugaga# go get github.com/szferi/gomdb
# github.com/szferi/gomdb
/usr/bin/ld: cannot find -llmdb
collect2: ld returned 1 exit status
```

However `gomdb` repository _already_ includes the source code for `lmdb`, and only a small change is needed to remove the external dependency on `lmdb.h` and `lmdb`. This pull request proposes this change.

After applying the supplied patch, it will be possible to perform `go get github.com/szferi/gomdb` without having lmdb headers / library installed on the machine, just as it is possible right now with `go get github.com/Babazka/gomdb`.