use environment flag HNSWLIB_NO_CFLAGS

Author: Tony Kuo

setup.py

@@ -77,6 +77,10 @@ class BuildExt(build_ext):
         #'unix': ['-O3', '-march=native'],  # , '-w'
         'unix': ['-O3'],  # , '-w'
     }
+    if not os.environ.get("HNSWLIB_NO_CFLAGS"):
+        if "conda" not in sys.version.lower():
+            c_opts['unix'].append('-march=native')
+
     link_opts = {
         'unix': [],
         'msvc': [],