Introduce builds for Linux, split archive into release and extras (#405)

## Link to GitHub Issue, if one exists
n/a

## Description of change
Add a "Linux" variant of spicecfg/spice/spice64.exe - currently these
are the same as Windows binaries but built without dependency on
WinSCard, which makes it easier to run on WINE.

Split dist archive into two, move api and extras into the new extras
zip.

## Testing
I only checked to see that Linux variants run fine on Windows, and
checked with dumpbin to confirm that the dependency on winscard DLL is
removed. No official support will be provided for Linux builds.

src/spice2x/overlay/windows/config.cpp

@@ -2904,10 +2904,17 @@ namespace overlay::windows {
     }
 
     void Config::build_about() {
+#ifndef SPICE_LINUX
         ImGui::TextUnformatted(std::string(
             "spice2x (a fork of SpiceTools)\r\n"
             "=========================\r\n" +
             to_string(VERSION_STRING)).c_str());
+#else
+        ImGui::TextUnformatted(std::string(
+            "spice2x (a fork of SpiceTools) for Linux\r\n"
+            "=========================\r\n" +
+            to_string(VERSION_STRING)).c_str());
+#endif
 
         ImGui::TextUnformatted("");
         if (ImGui::Button(PROJECT_URL)) {