Makefile.ci.in: Add make targets for OSS fuzzer
Makefile.ci.in: Add make targets for OSS fuzzer

This change extends the continuous integration Makefile.ci with
new rules for creating the executables for OSS-Fuzz [1].

Object files and linked executables are now created in the directory
$OUT which is set to '.' by default.

[1] https://github.com/google/oss-fuzz

authmillenon/libcoap,authmillenon/libcoap,authmillenon/libcoap