CMake: Add embedded Linux subprocess for WebView support

In order to display a WebKit based webview a plugin will deploy a
temporary standalone executable on the system and host the WebKit
instance inside that.

extras/Build/CMake/juce_LinuxSubprocessHelper.cpp

@@ -0,0 +1,36 @@
+/*
+  ==============================================================================
+
+   This file is part of the JUCE library.
+   Copyright (c) 2022 - Raw Material Software Limited
+
+   JUCE is an open source library subject to commercial or open-source
+   licensing.
+
+   By using JUCE, you agree to the terms of both the JUCE 7 End-User License
+   Agreement and JUCE Privacy Policy.
+
+   End User License Agreement: www.juce.com/juce-7-licence
+   Privacy Policy: www.juce.com/juce-privacy-policy
+
+   Or: You may also use this code under the terms of the GPL v3 (see
+   www.gnu.org/licenses).
+
+   JUCE IS PROVIDED "AS IS" WITHOUT ANY WARRANTY, AND ALL WARRANTIES, WHETHER
+   EXPRESSED OR IMPLIED, INCLUDING MERCHANTABILITY AND FITNESS FOR PURPOSE, ARE
+   DISCLAIMED.
+
+  ==============================================================================
+*/
+
+#include <dlfcn.h>
+
+int main (int argc, const char* const* argv)
+{
+    if (argc >= 3)
+        if (auto* handle = dlopen (argv[1], RTLD_LAZY))
+            if (auto* function = reinterpret_cast<int (*) (int, const char* const*)> (dlsym (handle, argv[2])))
+                return function (argc - 3, argv + 3);
+
+    return 1;
+}