build-sys: improve check for crypt

build-sys: improve check for crypt

Check for crypt.h existence, and use it if available over using unistd.h
for which a certain feature level has to be set to export a definition
for crypt.  On Solaris this set causes a standards conflict in the
headers, because at the time of this check C99 mode is already enabled,
which implies certain standards non-compatible with _XOPEN_SOURCE.

 92  #define _XOPEN_SOURCE
 93  #include <unistd.h>

configure:16259: gcc -std=gnu99 -c -g -O2  conftest.c >&5
In file included from /usr/include/unistd.h:18,
                 from conftest.c:93:
/prefix/gentoo/usr/lib/gcc/i386-pc-solaris2.10/4.4.5/include-fixed/sys/feature_tests.h:341:2: error: #error "Compiler or options invalid for pre-UNIX 03 X/Open applications and pre-2001 POSIX applications"

configure.ac: improve crypt check
login-utils/my_crypt.h: replace old GNU_LIBRARY check with autoconf
define for crypt.h

[kzak@redhat.com: - remove my_crypt.h]

Signed-off-by: Fabian Groffen <grobian@gentoo.org>
Signed-off-by: Karel Zak <kzak@redhat.com>