proc-macros: add BUILD files

Signed-off-by: Austin Seipp <[email protected]>

lib/proc-macros/BUILD

@@ -0,0 +1,22 @@
+
+load("//buck/shims/jj.bzl", "jj")
+
+alias(
+    # NOTE: default target for this package
+    name = 'proc-macros',
+    actual = ':jj-lib-proc-macros',
+)
+
+jj.rust_library(
+    name = 'jj-lib-proc-macros',
+    srcs = glob(["src/**/*.rs"]),
+    proc_macro = True,
+    deps = [
+        # CARGO-SYNC-START: dependencies
+        'third-party//rust:proc-macro2',
+        'third-party//rust:quote',
+        'third-party//rust:syn',
+        # CARGO-SYNC-END
+    ],
+    visibility = ['PUBLIC'], # XXX FIXME (aseipp): why is this needed?
+)