Always check -Wno-unused-but-set-variable gcc support

The previous commit 8a7e1ceefa430988c8f888ca708ab307333b4464 wasn't
quite right. This check applies to both the user and kernel space
build and as such we must make sure it runs regardless of what
the --with-config option is set too.

For example, if --with-config=kernel then the autoconf test does
not run and we generate build warnings when compiling the kernel
packages.