build: Fix Boost Process compatibility with mingw-w64 compiler

Boost 1.71 has a broken compatibility with mingw-w64 compiler due to the
added __kernel_entry SAL annotations.

Author: hebasto

src/test/system_tests.cpp

@@ -7,6 +7,11 @@
 #include <univalue.h>
 
 #ifdef ENABLE_EXTERNAL_SIGNER
+#if defined(WIN32) && !defined(__kernel_entry)
+// A workaround for boost 1.71 incompatibility with mingw-w64 compiler.
+// For details see https://github.com/bitcoin/bitcoin/pull/22348.
+#define __kernel_entry
+#endif
 #include <boost/process.hpp>
 #endif // ENABLE_EXTERNAL_SIGNER
 

src/util/system.cpp

@@ -6,6 +6,11 @@
 #include <util/system.h>
 
 #ifdef ENABLE_EXTERNAL_SIGNER
+#if defined(WIN32) && !defined(__kernel_entry)
+// A workaround for boost 1.71 incompatibility with mingw-w64 compiler.
+// For details see https://github.com/bitcoin/bitcoin/pull/22348.
+#define __kernel_entry
+#endif
 #include <boost/process.hpp>
 #endif // ENABLE_EXTERNAL_SIGNER