Meta: Add the start of a gn build based on the LLVM gn build

"based on" in this context means "largely copied from"

Meta/gn/build/libs/crypt/BUILD.gn

@@ -0,0 +1,10 @@
+config("crypt_config") {
+  visibility = [ ":crypt" ]
+  libs = [ "crypt" ]
+}
+
+group("crypt") {
+  if (current_os == "linux") {
+    public_configs = [ ":crypt_config" ]
+  }
+}

Meta/gn/build/libs/pthread/BUILD.gn

@@ -0,0 +1,11 @@
+config("pthread_config") {
+  visibility = [ ":pthread" ]
+  libs = [ "pthread" ]
+}
+
+group("pthread") {
+  # On Android, bionic has built-in support for pthreads.
+  if (current_os != "android") {
+    public_configs = [ ":pthread_config" ]
+  }
+}