configure.ac: check for the library containing fts_open

configure.ac: check for the library containing fts_open

The musl C library doesn't provide fts.h[1], so libcgroup doesn't
compile with musl. However, there is a standalone implementation of fts
for musl users[2] which can be used.

Use AC_SEARCH_LIBS to search for fts_open, which will check if it is part
of libc, and if not look in libfts, then set LIBS if needed.

[1] https://wiki.musl-libc.org/faq.html
[2] https://github.com/void-linux/musl-fts

Closes #61.

Signed-off-by: Ross Burton <ross.burton@arm.com>
[TJH: minor formatting changes to match existing configure.ac]
Signed-off-by: Tom Hromatka <tom.hromatka@oracle.com>