meson: fallback to libcrypt detection outside of pkg-config

Much like PAM, not all implementations of libcrypt provide a pkg-config
file, and hence we can't find it using the old logic.

Let's fall back to the standard AC_SEARCH_LIBS-style check if the pkg-config-style
detection fails.

This fixes finding e.g. musl's libcrypt.

X-Gentoo-Bug: 827074
X-Gentoo-Bug-URL: https://bugs.gentoo.org/827074

meson.build

@@ -108,7 +108,10 @@ if selinux_dep.found()
       error('Pam was requested but could not be located')
     endif
   else
-    crypt_dep = dependency('libcrypt', required : not get_option('pam'))
+    crypt_dep = dependency('libcrypt', required : false)
+    if not crypt_dep.found()
+      crypt_dep = cc.find_library('crypt', required : true)
+    endif
   endif
 else
   cc_selinux_flags = []