pkg/libcoap: fix copmilation problems

Apparently all that whitespace was needed after all.
Next convert to new-style function declrations.
Also, don't prevent make from noticing a failing patch.
tests: add libcoap compilation test
Merge pull request #1429 from LudwigOrtmann/libcoap-fixup

pkg/libcoap: fix build AND tests: add one to ensure it does not break in the future