Define _BSD_SOURCE for strdup and timerclear

Summary:
strdup is an extention to the C standard, timerclear originates from
the BSDs. GNU libc exposes both by default but musl libc requires a
feature flag to get them defined and thus fix libkcompactdisc's
build.

Test Plan: Builds fine with glibc and musl libcs.

Differential Revision: https://phabricator.kde.org/D19615