build: disable external-signer for Windows

It's come to light that Boost ASIO (a Boost Process sub dep) has in some
instances, been queitly initialising our network stack on Windows (see
PR #28486 and discussion in #28940).

This has been shielding a bug in our own code, but the larger issue
is that Boost Process/ASIO is running code before main, and doing things
like setting up networking. This undermines our own assumptions about
how our binary works, happens before we get to run any sanity checks,
and also runs before we call our own code to setup networking.

It's also not clear why a feature like external signer would have a
dependency that would be doing anything network/socket related, given it
only exists to spawn a local process.

build_msvc/bitcoin_config.h.in

@@ -41,9 +41,6 @@
 /* Define this symbol to enable ZMQ functions */
 #define ENABLE_ZMQ 1
 
-/* define if external signer support is enabled (requires Boost::Process) */
-#define ENABLE_EXTERNAL_SIGNER /**/
-
 /* Define to 1 if you have the declaration of `be16toh', and to 0 if you
    don't. */
 #define HAVE_DECL_BE16TOH 0

build_msvc/vcpkg.json

@@ -4,7 +4,6 @@
   "dependencies": [
     "berkeleydb",
     "boost-multi-index",
-    "boost-process",
     "boost-signals2",
     "boost-test",
     "sqlite3",

configure.ac

@@ -1520,9 +1520,19 @@ if test "$use_external_signer" != "no"; then
   CXXFLAGS="$TEMP_CXXFLAGS"
   AC_MSG_RESULT([$have_boost_process])
   if test "$have_boost_process" = "yes"; then
-    use_external_signer="yes"
-    AC_DEFINE([ENABLE_EXTERNAL_SIGNER], [1], [Define if external signer support is enabled])
-    AC_DEFINE([BOOST_PROCESS_USE_STD_FS], [1], [Defined to avoid Boost::Process trying to use Boost Filesystem])
+    case $host in
+      dnl Boost Process for Windows uses Boost ASIO. Boost ASIO performs
+      dnl pre-main init of Windows networking libraries, which we do not
+      dnl want.
+      *mingw*)
+        use_external_signer="no"
+      ;;
+      *)
+        use_external_signer="yes"
+        AC_DEFINE([ENABLE_EXTERNAL_SIGNER], [1], [Define if external signer support is enabled])
+        AC_DEFINE([BOOST_PROCESS_USE_STD_FS], [1], [Defined to avoid Boost::Process trying to use Boost Filesystem])
+      ;;
+    esac
   else
     if test "$use_external_signer" = "yes"; then
       AC_MSG_ERROR([External signing is not supported for this Boost version])