Add support for meson build system.

For now, let's keep support for both - autotools and meson.

lib/crypto_backend/argon2/meson.build

@@ -0,0 +1,28 @@
+libargon2_sources = files(
+    'blake2/blake2b.c',
+    'argon2.c',
+    'core.c',
+    'encoding.c',
+    'thread.c',
+)
+
+if use_internal_sse_argon2
+    libargon2_sources += files(
+        'opt.c',
+    )
+else
+    libargon2_sources += files(
+        'ref.c',
+    )
+endif
+
+libargon2 = static_library('argon2',
+    libargon2_sources,
+    override_options : ['c_std=c89', 'optimization=3'],
+    build_by_default : false,
+    include_directories: include_directories(
+        'blake2',
+    ),
+    dependencies : [
+        threads,
+    ])