It is useful when somebody has different Go versions across one system
and first go in $PATH points to version older than 1.12 (common in
stable distributions).

Signed-off-by: Kacper Kołodziej <kacper@kolodziej.it>

Aerc uses the go linker to add some variables like main.Prefix / main.ShareDir
etc. however the go linker only considers the last -ldflags invocation it seems.

This commit fixes the makefile to use only a single -ldflags invocation, which
again sets the vars as expected.

Also removes leftover directories if there isn't anything else in them.

https://todo.sr.ht/~sircmpwn/aerc2/179