meson: allow to fallback to use libxcrypt.pc or glibc's libcrypt

Some distributions still use glibc's libcrypt. In that case, libcrypt.pc
does not exist and dependency() will fail.
Also, even if libxcrypt is used, there may not be a symlink
from libcrypt.pc to libxcrypt.pc. So, let's add a secondary name.

Follow-up for d625f71.

Fixes #28289.

[ fixed to fallback to extra dependency() call as multiple deps require meson 0.60 ]

(cherry picked from commit 5557378)
(cherry picked from commit 087b9a7)
(cherry picked from commit 6f7b7b6)

meson.build

@@ -998,7 +998,14 @@ threads = dependency('threads')
 librt = cc.find_library('rt')
 libm = cc.find_library('m')
 libdl = cc.find_library('dl')
-libcrypt = dependency('libcrypt')
+libcrypt = dependency('libcrypt', required : false)
+if not libcrypt.found()
+        libcrypt = dependency('libxcrypt', required : false)
+endif
+if not libcrypt.found()
+        # fallback to use find_library() if libcrypt is provided by glibc, e.g. for LibreELEC.
+        libcrypt = cc.find_library('crypt')
+endif
 libcap = dependency('libcap')
 
 # On some architectures, libatomic is required. But on some installations,