build: avoid using args.user_cflags unconditionally
build: avoid using args.user_cflags unconditionally

We'll want to run try_compile() soon before user arguments are passed,
to be able to supply sensible defaults. To do that, pass args.user_cflags
explicitly everywhere.

syuu1228/seastar,scylladb/seastar,syuu1228/seastar,cloudius-systems/seastar,cloudius-systems/seastar,dreamsxin/seastar,scylladb/seastar,avikivity/seastar,cloudius-systems/seastar,syuu1228/seastar,dreamsxin/seastar,avikivity/seastar,scylladb/seastar,dreamsxin/seastar,avikivity/seastar