message: doc: recommend `--disable-external-signer` in OpenBSD build guide
diff: OpenBSD build guide
  ======================
  -(updated for OpenBSD 6.7)
  +(updated for OpenBSD 6.9)
  This guide describes how to build peercoind, peercoin-qt, and command-line utilities on OpenBSD.
  @@ -67,9 +67,16 @@ export AUTOMAKE_VERSION=1.16
  ```
  Make sure `BDB_PREFIX` is set to the appropriate path from the above steps.
  +Note that building with external signer support currently fails on OpenBSD,
  +hence you have to explicitely disable it by passing the parameter
  +`--disable-external-signer` to the configure script.
  +(Background: the feature requires the header-only library boost::process, which
  +is available on OpenBSD 6.9 via Boost 1.72.0, but contains certain system calls
  +and preprocessor defines like `waitid()` and `WEXITED` that are not available.)
  +
  To configure with wallet:
  ```bash
  -./configure --with-gui=no CC=cc CXX=c++ \
  +./configure --with-gui=no --disable-external-signer CC=cc CXX=c++ \
  BDB_LIBS="-L${BDB_PREFIX}/lib -ldb_cxx-4.8" \
  BDB_CFLAGS="-I${BDB_PREFIX}/include" \
  MAKE=gmake
  @@ -77,12 +84,12 @@ To configure with wallet:
  To configure without wallet:
  ```bash
  -./configure --disable-wallet --with-gui=no CC=cc CC_FOR_BUILD=cc CXX=c++ MAKE=gmake
  +./configure --disable-wallet --with-gui=no --disable-external-signer CC=cc CC_FOR_BUILD=cc CXX=c++ MAKE=gmake
  ```
  To configure with GUI:
  ```bash
  -./configure --with-gui=yes CC=cc CXX=c++ \
  +./configure --with-gui=yes --disable-external-signer CC=cc CXX=c++ \
  BDB_LIBS="-L${BDB_PREFIX}/lib -ldb_cxx-4.8" \
  BDB_CFLAGS="-I${BDB_PREFIX}/include" \
  MAKE=gmake
  