CFLAGS picked up from environment are overridden by mkspecs

My CFLAGS:
CFLAGS="-march=native -O2 -pipe"

Output:
x86_64-pc-linux-gnu-g++ -c -pipe -mmmx -march=native -O2 -pipe -std=c++0x -O2 -Wall -W  -I../../../mkspecs/linux-g++ -I. -I/usr/include/freetype2 -o mmx.o mmx.cpp
x86_64-pc-linux-gnu-g++ -Wl,-O1 -Wl,--as-needed -Wl,-O1 -o mmx mmx.o     -L/usr/lib64 -mmmx 
mmx enabled.
3dnow auto-detection... ()
x86_64-pc-linux-gnu-g++ -c -pipe -m3dnow -march=native -O2 -pipe -std=c++0x -O2 -Wall -W  -I../../../mkspecs/linux-g++ -I. -I/usr/include/freetype2 -o 3dnow.o 3dnow.cpp
x86_64-pc-linux-gnu-g++ -Wl,-O1 -Wl,--as-needed -Wl,-O1 -o 3dnow 3dnow.o     -L/usr/lib64 -m3dnow 
[...]


Same on another system:
i686-pc-linux-gnu-g++ -c -pipe -mmmx -Os -g0 -pipe -O2 -Wall -W  -I../../../mkspecs/linux-g++ -I. -I/usr/include/freetype2 -o mmx.o mmx.cpp
i686-pc-linux-gnu-g++ -Wl,-O1 -Wl,--as-needed -Wl,-O1 -o mmx mmx.o     -mmmx 
mmx enabled.
3dnow auto-detection... ()
i686-pc-linux-gnu-g++ -c -pipe -m3dnow -Os -g0 -pipe -O2 -Wall -W  -I../../../mkspecs/linux-g++ -I. -I/usr/include/freetype2 -o 3dnow.o 3dnow.cpp
i686-pc-linux-gnu-g++ -Wl,-O1 -Wl,--as-needed -Wl,-O1 -o 3dnow 3dnow.o     -m3dnow 
[...]