build: don't perform test compiles to /dev/null
build: don't perform test compiles to /dev/null

With some cflags (e.g. -ftest-coverage) gcc generates an auxiliary output
file based on the main output file name. If the main output file name is
/dev/null, the auxiliary file name will be one that gcc can't create, and
the compilation will fail.

Fix by creating a valid temporary name for the output file, that gcc can
create valid derivations off.

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