musl doesn't support fts. But with the extra package musl-fts installed,
libzip picks up the fts header and fails at the linking stage:

zipcmp.c:(.text.startup+0x130): undefined reference to `fts_open'
/home/sk/tmp/openwrt/staging_dir/toolchain-mips_24kc_gcc-8.3.0_musl/lib/gcc/mips-openwrt-linux-musl/8.3.0/../../../../mips-openwrt-linux-musl/bin/ld: zipcmp.c:(.text.startup+0x172): undefined reference to `fts_read'

So with musl-fts we need to link in libfts. To address that this commits
patches the cmake setup to check if fts is available in libc itself or
in any external libfts.

So when musl-fts is installed on the system the setup will be the
following:

musl: use libfts
uclibc: use fts from libc
glibc: like uclibc

Signed-off-by: Sebastian Kemper <sebastian_ml@gmx.net>