build-all: unset standard flags on beginning
build-all: unset standard flags on beginning

Using CFLAGS or some other standard environment variables is risky.
Unset them all and rely instead on some custom variable, i.e. expect
their name to be prefixed by AFM_ (AFM_CFLAGS, AFM_LDFLAGS and so on).

ntd/aur-fedora-mingw