Duplicate report from Stephan Krah:

When CFLAGS are set, distutils drops -fno-strict-aliasing (among other
flags):


$ python2.7 setup.py build
...
gcc -pthread -fno-strict-aliasing -g -O2 -DNDEBUG -g -fwrapv -O3 -Wall -Wstrict-prototypes -fPIC -I./src -I/usr/local/include -I/usr/local/include/python2.7 -c src/gmpy.c -o build/temp.linux-x86_64-2.7/src/gmpy.o
...

$ CFLAGS="-fomit-frame-pointer" python2.7 setup.py build
...
gcc -pthread -DNDEBUG -g -fwrapv -O3 -Wall -Wstrict-prototypes -fomit-frame-pointer -fPIC -I./src -I/usr/local/include -I/usr/local/include/python2.7 -c src/gmpy.c -o build/temp.linux-x86_64-2.7/src/gmpy.o
src/gmpy.c: In function ‘_cmp_to_object’:
src/gmpy.c:4692: warning: dereferencing type-punned pointer will break strict-aliasing rules
...

I'm not sure if this is intentional. The documentation says:

"Compiler flags can also be supplied through setting the CFLAGS
environment variable. If set, the contents of CFLAGS will be added
to the compiler flags specified in the Setup file."

To me, this sounds as if they should be appended to the regular flags.