Add DIFF_ENGINE_VERSION macro for pip builds

The diff_engine_core uses DIFF_ENGINE_VERSION macro (defined in CMakeLists.txt
for CMake builds). Added -DDIFF_ENGINE_VERSION="0.0.1" to compile args for
setuptools/pip builds.

Cherry-picked from submodule-setup branch.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: Transurgeon

setup.py

@@ -55,8 +55,9 @@ def finalize_options(self) -> None:
 extensions = [
     setup_extensions.cvxcore,
     setup_extensions.sparsecholesky,
-    setup_extensions.diffengine,  # None if submodule not initialized
+    setup_extensions.diffengine,
 ]
+# Filter out None (diffengine is None if submodule not initialized)
 extensions = [ext for ext in extensions if ext is not None]
 
 setup(

setup/extensions.py

@@ -92,6 +92,7 @@ def not_on_windows(s: str) -> str:
             '-std=c99',
             '-Wall',
             not_on_windows('-Wextra'),
+            '-DDIFF_ENGINE_VERSION="0.0.1"',
         ],
         extra_link_args=['-lm'] if platform.system().lower() != 'windows' else [],
     )