Merge bitcoin/bitcoin#26073: build: fix depends bdb compilation for BSDs

5b2529b2697e9b2675783c98a32a41e28c72e076 build: fix depends bdb compilation for BSDs (fanquake)

Pull request description:

  Currently, building bdb for *BSD HOSTs in depends fails with:
  ```bash
  libtool: compile:  clang -m64 -c -I. -I../dist/./.. -I/home/ubuntu/bitcoin/depends/x86_64-unknown-freebsd/include -D_THREAD_SAFE -pipe -std=c11 -O2 -Wno-error=implicit-function-declaration -Wno-error=format-security ../dist/./../mutex/mut_tas.c  -fPIC -DPIC -o mut_tas.o
  In file included from ../dist/./../mutex/mut_tas.c:11:
  In file included from ./db_int.h:884:
  In file included from ../dist/./../dbinc/mutex.h:15:
  ../dist/./../dbinc/mutex_int.h:932:2: error: unknown type name 'pthread_rwlock_t'
          MUTEX_FIELDS                    /* Opaque thread mutex structures. */
          ^
  ../dist/./../dbinc/mutex_int.h:65:3: note: expanded from macro 'MUTEX_FIELDS'
                  pthread_rwlock_t rwlock;        /* Read/write lock */   \
                  ^
  1 error generated.
  ```

  Defining `_XOPEN_SOURCE` >=600 fixes access to the missing `pthread_rwlock_t` definitions.

ACKs for top commit:
  jarolrod:
    ACK 5b2529b2697e9b2675783c98a32a41e28c72e076

Tree-SHA512: 76584bfef7a86c69a095eb722657aa681b95658a031ef2da79d91a616e5370da292a65e7d67cdf641711791422b8a407b647b630ad497ffb9908683479e6cfb6

0 files changed, 0 insertions, 0 deletions