setup: Honor CFLAGS set as environment variable.

Distributions normally have their preferred set of CFLAGS so the
binaries are optimized and include debug symbols to create a debuginfo
package. For this we need to be able to set the CFLAGS via the
commandline and the setup.py should forward them to the compiler.

Signed-off-by: Andreas Schneider <asn@cryptomilk.org>