graphics: remove libjpeg from XP builds, fetch at build time (#874)

Replaces the vendored `external/libjpeg-turbo` tree (191 files) with the
`mingw-w64-libjpeg-turbo` AUR package, already built into the Docker
deps image.

The WinXP toolchains set `CMAKE_FIND_ROOT_PATH_MODE_LIBRARY ONLY`
against their own sysroot, so they cannot see AUR mingw packages. Those
targets now build without JPEG support: `jpeg_encoder::encode` compiles
to a stub returning `false`, which callers already treat as "no frame
available". The practical effect is that `capture.get_jpg` over the API
is unavailable on WinXP builds; every other target is unchanged.

CMake names `libjpeg.a` outright, since the package also ships an import
library and linking that would add a runtime DLL dependency.

src/spice2x/hooks/graphics/jpeg_encoder.cpp

@@ -1,5 +1,7 @@
 #include "jpeg_encoder.h"
 
+#ifdef SPICE_JPEG
+
 #include <csetjmp>
 #include <cstdio>
 
@@ -138,3 +140,17 @@ bool encode(
     return true;
 }
 }
+
+#else // SPICE_JPEG
+
+namespace jpeg_encoder {
+
+// builds without libjpeg-turbo (the WinXP toolchains) simply cannot encode;
+// callers already treat a false return as "no frame available"
+bool encode(std::vector<uint8_t> &out, const uint8_t *, int, int, int) {
+    out.clear();
+    return false;
+}
+}
+
+#endif // SPICE_JPEG