libtoxcore: don't set PKG_CONFIG_PATH

This is automatically set by Homebrew.
libtoxcore: style nits
toxic: use Homebrew-built flags

CFLAGS and LDFLAGS are automatically set by Homebrew; use that instead
of hardcoding paths.
utox: use Homebrew-built flags
libtoxcore: fix option description
Merge branch 'mistydemeo-paths'