[cmake] root wrapper

allow the existence of a `CMakeLists.txt` file at root,
for easier integration with other projects expecting this file at root.

Existing integration point, within `build/cmake/`, still works as expected.

CMakeLists.txt

@@ -0,0 +1,11 @@
+cmake_minimum_required(VERSION 3.10)
+
+# Thin wrapper so `cmake -S .` behaves like `cmake -S build/cmake`.
+# Policy lives in build/cmake; keep parent project language-less.
+project(zstd-superbuild LANGUAGES NONE)
+
+if(CMAKE_SOURCE_DIR STREQUAL CMAKE_BINARY_DIR)
+    message(FATAL_ERROR "In-source builds are not supported. Specify -B <build-dir>.")
+endif()
+
+add_subdirectory(build/cmake)