fio: fix "error: Define GOLDEN_RATIO_PRIME"

Let fio's own CFLAGS settings stand instead of clearing them in the formula.

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.

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 #19380.