m4: Check for FTW_ACTIONRETVAL along with nftw

m4: Check for FTW_ACTIONRETVAL along with nftw

FTW_ACTIONRETVAL is glibc specific extention which is used to implement
xfsfind but it may not be available on other C library implementations on Linux
e.g. musl. Therefore ensure that these defines are available before declaring
nftw() to be usable

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Cc: Darrick J. Wong <djwong@kernel.org>
Cc: Zorro Lang <zlang@redhat.com>
Acked-by: Darrick J. Wong <djwong@kernel.org>