Handle absent <crypt.h>

The prototype for crypt(3) is in <unistd.h> on Linux and FreeBSD.
The <crypt.h> header isn't needed at all. There's also machinery
for detecting the presence of <crypt.h>, defining HAVE_CRYPT_H,
but we don't need that.

Reported and resolved independently in
	https://phabricator.kde.org/D29847

@nicofe tested that this works on Linux
@adridg tested that this works on FreeBSD