fio 2.0.15: fix "error: Define GOLDEN_RATIO_PRIME"

The new 2.0.15 fio build was breaking because the Makefile changed around since
2.0.7, and is now using CFLAGS to pass necessary macros and "-include" options
to the build. The formula was crushing it with "CFLAGS=", which broke the build.

Let fio's own CFLAGS settings stand. Clear OPTFLAGS and remove some "-W"s from the
Makefile to avoid superenv's "removed: ..." output.

Also turn on "V=1" to get normal full output from make, showing cc command lines,
instead of the terse "CC foo.o" output, which makes it harder to diagnose.

Fixes https://github.com/mxcl/homebrew/issues/19380