Do not overwrite user's optimization.

Bug: https://bugs.gentoo.org/887651

--- a/setup.py
+++ b/setup.py
@@ -27,7 +27,7 @@ from setuptools.command import build_ext as setuptools_build_ext
 
 CYTHON_DEPENDENCY = 'Cython(>=0.29.24,<4.0.0)'
 
-CFLAGS = ['-O2']
+CFLAGS = []
 LDFLAGS = []
 
 if platform.uname().system != 'Windows':