Makefile: Simplify setting of CFLAGS, etc.

We were previously using separate CFLAGS and NOTMUCH_CFLAGS variables
in an attempt to allow the user to specify CFLAGS on the command-line.

However, that's just a lot of extra noise in the Makefile when we can
instead let the user specify what is desired for CFLAGS and then use
an override to append the things we require. So our Makefile is much
neater now.