configure: Detect compatibility of Boost.Process rather than hardcode non-Windows

configure.ac

@@ -1493,17 +1493,6 @@ if test "$use_boost" = "yes"; then
 fi
 
 if test "$use_external_signer" != "no"; then
-  case $host in
-    *mingw*)
-      dnl Boost Process uses Boost Filesystem when targeting Windows. Also,
-      dnl since Boost 1.71.0, Process does not work with mingw-w64 without
-      dnl workarounds. See 67669ab425b52a2b6be3d2f3b3b7e3939b676a2c.
-      if test "$use_external_signer" = "yes"; then
-        AC_MSG_ERROR([External signing is not supported on Windows])
-      fi
-      use_external_signer="no";
-    ;;
-    *)
   AC_MSG_CHECKING([whether Boost.Process can be used])
   TEMP_CXXFLAGS="$CXXFLAGS"
   dnl Boost 1.78 requires the following workaround.
@@ -1514,7 +1503,19 @@ if test "$use_external_signer" != "no"; then
   TEMP_LDFLAGS="$LDFLAGS"
   dnl Boost 1.73 and older require the following workaround.
   LDFLAGS="$LDFLAGS $PTHREAD_CFLAGS"
-      AC_LINK_IFELSE([AC_LANG_PROGRAM([[#include <boost/process.hpp>]])],
+  AC_LINK_IFELSE([AC_LANG_PROGRAM([[
+    #define BOOST_PROCESS_USE_STD_FS
+    #include <boost/process.hpp>
+  ]],[[
+    namespace bp = boost::process;
+    bp::opstream stdin_stream;
+    bp::ipstream stdout_stream;
+    bp::child c("dummy", bp::std_out > stdout_stream, bp::std_err > stdout_stream, bp::std_in < stdin_stream);
+    stdin_stream << std::string{"test"} << std::endl;
+    if (c.running()) c.terminate();
+    c.wait();
+    c.exit_code();
+  ]])],
     [have_boost_process="yes"],
     [have_boost_process="no"])
   LDFLAGS="$TEMP_LDFLAGS"
@@ -1524,14 +1525,13 @@ if test "$use_external_signer" != "no"; then
   if test "$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])
   else
     if test "$use_external_signer" = "yes"; then
       AC_MSG_ERROR([External signing is not supported for this Boost version])
     fi
     use_external_signer="no";
   fi
-    ;;
-  esac
 fi
 AM_CONDITIONAL([ENABLE_EXTERNAL_SIGNER], [test "$use_external_signer" = "yes"])