pppd: Use a compile test to detect crypt.h (#198)

ppp checks header for existence of crypt.h looking it up in /usr/include.
That's incompatible with non-glibcs or a glibc with multiarch headers
(https://bugs.debian.org/798955). This patch replaces the file existence
test with a compile test.

Reviewed-by: Chris Boot <bootc@debian.org>
Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Co-authored-by: Helmut Grohne <helmut@subdivi.de>