libcrypt-util: use build-time check for crypt_preferred_method

After https://github.com/systemd/systemd/pull/16981 only the presence of crypt_gensalt_ra
is checked, but there are cases where that function is available but crypt_preferred_method
is not, and they are used in the same ifdef.
Add a check for the latter as well.

meson.build

@@ -884,8 +884,9 @@ libcrypt = cc.find_library('crypt')
 
 crypt_header = conf.get('HAVE_CRYPT_H') == 1 ? '''#include <crypt.h>''' : '''#include <unistd.h>'''
 foreach ident : [
-        ['crypt_ra',          crypt_header],
-        ['crypt_gensalt_ra',  crypt_header]]
+        ['crypt_ra',               crypt_header],
+        ['crypt_preferred_method', crypt_header],
+        ['crypt_gensalt_ra',       crypt_header]]
 
         have = cc.has_function(ident[0], prefix : ident[1], args : '-D_GNU_SOURCE',
                                dependencies : libcrypt)

src/shared/libcrypt-util.c

@@ -40,7 +40,11 @@ int make_salt(char **ret) {
 
         e = secure_getenv("SYSTEMD_CRYPT_PREFIX");
         if (!e)
+#if HAVE_CRYPT_PREFERRED_METHOD
                 e = crypt_preferred_method();
+#else
+                e = "$6$";
+#endif
 
         log_debug("Generating salt for hash prefix: %s", e);