Do not overwrite user's optimization.

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

--- a/setup.py
+++ b/setup.py
@@ -29,7 +29,7 @@
 
 CYTHON_DEPENDENCY = 'Cython>=3.2.1,<4.0.0'
 
-CFLAGS = ['-O2']
+CFLAGS = []
 LDFLAGS = []
 
 if platform.uname().system != 'Windows':