contrib/credential/osxkeychain: Makefile should allow to set CFLAGS

Hi there,

the contrib/credential/osxkeychain/Makefile goes like this

CC = gcc
RM = rm -f
CFLAGS = -g -Wall

git-credential-osxkeychain: git-credential-osxkeychain.o
	$(CC) -o $@ $< -Wl,-framework -Wl,Security

and does not allow me to pass CFLAGS to build git-credential-osxkeychain.o.
My hack is to call: make CC="$CC $CFLAGS"
That is ugly.

Would you mind to add the CFLAGS to that CC call?

bests,
 Samuel