Merge bitcoin/bitcoin#22335: doc: recommend `--disable-external-signer` in OpenBSD build guide

e65d1d49864d047764eb2b444b2fc806b67e051c doc: recommend `--disable-external-signer` in OpenBSD build guide (Sebastian Falbesoner)

Pull request description:

  Building the master branch with the default build settings (i.e. with external signer support enabled) leads to the following errors on my OpenBSD 6.9 machine:

  ```
  In file included from util/system.cpp:9:
  In file included from /usr/local/include/boost/process.hpp:25:
  In file included from /usr/local/include/boost/process/group.hpp:32:
  /usr/local/include/boost/process/detail/posix/wait_group.hpp:38:17: error: no member named 'waitid' in the global namespace
          ret = ::waitid(P_PGID, p.grp, &status, WEXITED | WNOHANG);
                ~~^
  /usr/local/include/boost/process/detail/posix/wait_group.hpp:38:24: error: use of undeclared identifier 'P_PGID'
          ret = ::waitid(P_PGID, p.grp, &status, WEXITED | WNOHANG);
                         ^
  /usr/local/include/boost/process/detail/posix/wait_group.hpp:38:48: error: use of undeclared identifier 'WEXITED'
          ret = ::waitid(P_PGID, p.grp, &status, WEXITED | WNOHANG);
                                                 ^
  /usr/local/include/boost/process/detail/posix/wait_group.hpp:144:17: error: no member named 'waitid' in the global namespace
          ret = ::waitid(P_PGID, p.grp, &siginfo, WEXITED | WSTOPPED | WNOHANG);
                ~~^
  /usr/local/include/boost/process/detail/posix/wait_group.hpp:144:24: error: use of undeclared identifier 'P_PGID'
          ret = ::waitid(P_PGID, p.grp, &siginfo, WEXITED | WSTOPPED | WNOHANG);
                         ^
  /usr/local/include/boost/process/detail/posix/wait_group.hpp:144:49: error: use of undeclared identifier 'WEXITED'
          ret = ::waitid(P_PGID, p.grp, &siginfo, WEXITED | WSTOPPED | WNOHANG);
                                                  ^
  /usr/local/include/boost/process/detail/posix/wait_group.hpp:144:59: error: use of undeclared identifier 'WSTOPPED'
          ret = ::waitid(P_PGID, p.grp, &siginfo, WEXITED | WSTOPPED | WNOHANG);
                                                            ^
  7 errors generated.
  ```

  This PR recommends passing `--disable-external-signer` in the OpenBSD build guide ([as suggested by laanwj](https://github.com/bitcoin/bitcoin/pull/22294#issuecomment-867452411)). The same commit also bumps the OpenBSD version mentioned in the header to 6.9 -- I recently used this document to setup a Bitcoin Core build on 6.9 and the description and all mentioned versions were still valid (before external signer support was enabled by default).

  Would be nice if another OpenBSD user could confirm the build error.

ACKs for top commit:
  laanwj:
    ACK e65d1d49864d047764eb2b444b2fc806b67e051c

Tree-SHA512: c3ae7eca29cf42b4b52024477e1c3fb7242bbf9d809bc95f8fa08b2f9bf4bcfd4f22457d58569a208ac1d8e5fe41b270addd13d85a5bba0521a0f9e325288448

0 files changed, 0 insertions, 0 deletions