simplify initial dirs/macros setup

setup.py

@@ -142,19 +142,14 @@ cmdclass = {
 
 ext_modules = []
 if not on_rtd:
-    include_dirs = []
-    library_dirs = []
-    define_macros = []
-    compress_ext_kwargs = dict(sources=[compress_source], include_dirs=include_dirs, library_dirs=library_dirs,
-                               define_macros=define_macros)
+    compress_ext_kwargs = dict(sources=[compress_source])
     compress_ext_kwargs = setup_lz4.lz4_ext_kwargs(bundled_path='src/borg/algorithms/lz4',
                                                    prefer_system=prefer_system_liblz4,
                                                    **compress_ext_kwargs)
     compress_ext_kwargs = setup_zstd.zstd_ext_kwargs(bundled_path='src/borg/algorithms/zstd',
                                                      prefer_system=prefer_system_libzstd,
                                                      multithreaded=False, legacy=False, **compress_ext_kwargs)
-    crypto_ext_kwargs = dict(sources=[crypto_ll_source, crypto_helpers], include_dirs=include_dirs,
-                             library_dirs=library_dirs, define_macros=define_macros)
+    crypto_ext_kwargs = dict(sources=[crypto_ll_source, crypto_helpers])
     crypto_ext_kwargs = setup_crypto.crypto_ext_kwargs(**crypto_ext_kwargs)
     crypto_ext_kwargs = setup_b2.b2_ext_kwargs(bundled_path='src/borg/algorithms/blake2',
                                                prefer_system=prefer_system_libb2,