Enable existing MSVC stack backtrace code on MinGW where supported.

The existing code seems to work fine, though symbol lookup in
openttd.exe does not work.

config.lib

@@ -1612,6 +1612,27 @@ EOL
 		rm tmp.config.bfd
 	fi
 
+	if [ "$os" = "MINGW" ]; then
+		"$cc_host" $CFLAGS $LDFLAGS -o tmp.config.dbghelp -x c++ - 2> /dev/null << EOL
+			#include <windows.h>
+			#include <dbghelp.h>
+			int main() {
+				STACKFRAME64 frame;
+				IMAGEHLP_SYMBOL64 *sym_info;
+				IMAGEHLP_MODULE64 module;
+				IMAGEHLP_LINE64 line;
+				return 0;
+			}
+EOL
+		if [ $? -ne 0 ]; then
+			log 1 "checking dbghelp... no"
+		else
+			log 1 "checking dbghelp... found"
+			CFLAGS="$CFLAGS -DWITH_DBGHELP"
+		fi
+		rm -f tmp.config.dbghelp
+	fi
+
 	if [ "$os" != "CYGWIN" ] && [ "$os" != "HAIKU" ] && [ "$os" != "MINGW" ] && [ "$os" != "DOS" ] && [ "$os" != "WINCE" ]; then
 		LIBS="$LIBS -lc"
 	fi

src/os/windows/crashlog_win.cpp

@@ -49,12 +49,13 @@ class CrashLogWindows : public CrashLog {
 	/* virtual */ char *LogRegisters(char *buffer, const char *last) const;
 	/* virtual */ char *LogModules(char *buffer, const char *last) const;
 public:
-#if defined(_MSC_VER)
+#if defined(_MSC_VER) || defined(WITH_DBGHELP)
 	/* virtual */ int WriteCrashDump(char *filename, const char *filename_last) const;
 	char *AppendDecodedStacktrace(char *buffer, const char *last) const;
 #else
 	char *AppendDecodedStacktrace(char *buffer, const char *last) const { return buffer; }
-#endif /* _MSC_VER */
+#endif /* _MSC_VER || WITH_DBGHELP */
+
 
 	/** Buffer for the generated crash log */
 	char crashlog[65536];
@@ -321,10 +322,14 @@ static char *PrintModuleInfo(char *output, const char *last, HMODULE mod)
 	return buffer + seprintf(buffer, last, "\n");
 }
 
+#if defined(_MSC_VER) || defined(WITH_DBGHELP)
 #if defined(_MSC_VER)
 #pragma warning(disable:4091)
+#endif
 #include <dbghelp.h>
+#if defined(_MSC_VER)
 #pragma warning(default:4091)
+#endif
 
 char *CrashLogWindows::AppendDecodedStacktrace(char *buffer, const char *last) const
 {
@@ -479,7 +484,7 @@ char *CrashLogWindows::AppendDecodedStacktrace(char *buffer, const char *last) c
 	}
 	return ret;
 }
-#endif /* _MSC_VER */
+#endif /* _MSC_VER  || WITH_DBGHELP */
 
 extern bool CloseConsoleLogIfActive();
 static void ShowCrashlogWindow();