fix, PEBKAC, add -lm to libs, travis/drone (i.e. gcc) complain about undefined sqrt()

Signed-off-by: Sebastian Freundt <hroptatyr@fresse.org>
Merge branch 'bld/dist'

* bld/dist:
  fix, PEBKAC, add -lm to libs, travis/drone (i.e. gcc) complain about undefined sqrt()