lmdb: Silence missing initializers error on clang

[lmb]

Compiling this package on clang currently generates the following bogus warnings:

$ go test ./lmdb
# github.com/bmatsuo/lmdb-go/lmdb
lmdb/mdb.c:9100:20: warning: missing field 'mc_backup' initializer [-Wmissing-field-initializers]
lmdb/mdb.c:9258:18: warning: missing field 'mc_txn' initializer [-Wmissing-field-initializers]
ok      github.com/bmatsuo/lmdb-go/lmdb    0.380s

Add a flag to the CGO implementation which suppresses this output. See https://bugs.llvm.org//show_bug.cgi?id=21689 for the upstream bug.

[bmatsuo]

Looks good. Thanks @lmb