with a couple exceptions as they cause too many -Wshadow warnings.
also moves the `extcmd_t` typedef on top for cosmetic purposes.
also enable `-Wmissing-prototypes` in the ci
remove some non-posix extensions which slipped through and adjust ci to
new Makefile changes
users can still overwrite the variables explicitly by using
`make VAR=VALUE`
packagers can also add extra libs to LDLIBS, we're internally using
NSXIV_LDLIBS now.
* [ci] fix broken ci
* [ci] enable higher optimization level and lto
higher optimization levels enable more warnings and deeper analysis.
likewise, lto can catch a couple errors which typically goes unnoticed
without it.
