From 162608cc982538906e775d0c549aff2de5dee413 Mon Sep 17 00:00:00 2001
From: Hennadii Stepanov <32963518+hebasto@users.noreply.github.com>
Date: Thu, 20 Apr 2023 23:12:28 +0100
Subject: [PATCH] cmake: Emulate `PROJECT_IS_TOP_LEVEL` for CMake<3.21

---
 CMakeLists.txt | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 91d2bed0..0c8f049a 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -12,6 +12,18 @@ endif()
 # backwards-compatible and therefore at most increase the minor version.
 project(libsecp256k1 VERSION 0.3.2 LANGUAGES C)
 
+if(CMAKE_VERSION VERSION_LESS 3.21)
+  get_directory_property(parent_directory PARENT_DIRECTORY)
+  if(parent_directory)
+    set(PROJECT_IS_TOP_LEVEL OFF CACHE INTERNAL "Emulates CMake 3.21+ behavior.")
+    set(${PROJECT_NAME}_IS_TOP_LEVEL OFF CACHE INTERNAL "Emulates CMake 3.21+ behavior.")
+  else()
+    set(PROJECT_IS_TOP_LEVEL ON CACHE INTERNAL "Emulates CMake 3.21+ behavior.")
+    set(${PROJECT_NAME}_IS_TOP_LEVEL ON CACHE INTERNAL "Emulates CMake 3.21+ behavior.")
+  endif()
+  unset(parent_directory)
+endif()
+
 # The library version is based on libtool versioning of the ABI. The set of
 # rules for updating the version can be found here:
 # https://www.gnu.org/software/libtool/manual/html_node/Updating-version-info.html