Merge #1529: cmake: Fix cache issue when integrating by downstream project

ec4c002 cmake: Simplify `PROJECT_IS_TOP_LEVEL` emulation (Hennadii Stepanov)
cae9a7a cmake: Do not set emulated PROJECT_IS_TOP_LEVEL as cache variable (Hennadii Stepanov)

Pull request description:

  As CMake's cache is a global database, modifying it within a project integrated with the `add_subdirectory()` command, which may also include using the `FetchContent` module, could potentially affect downstream projects and sibling ones.

ACKs for top commit:
  real-or-random:
    utACK ec4c002
  theuni:
    utACK ec4c002

Tree-SHA512: de2c8c583367028d06701f79fc5232b351622c8496d196aad8c22a1ec4e450af53e556a4f6526ed47250f818143a69a12f5fc8cc755c864510e67530dacde66e

Author: real-or-random

CMakeLists.txt

@@ -18,15 +18,14 @@ project(libsecp256k1
 )
 
 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(${PROJECT_NAME}_IS_TOP_LEVEL OFF CACHE INTERNAL "Emulates CMake 3.21+ behavior.")
+  # Emulates CMake 3.21+ behavior.
+  if(CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR)
+    set(PROJECT_IS_TOP_LEVEL ON)
+    set(${PROJECT_NAME}_IS_TOP_LEVEL ON)
   else()
-    set(PROJECT_IS_TOP_LEVEL ON CACHE INTERNAL "Emulates CMake 3.21+ behavior.")
-    set(${PROJECT_NAME}_IS_TOP_LEVEL ON CACHE INTERNAL "Emulates CMake 3.21+ behavior.")
+    set(PROJECT_IS_TOP_LEVEL OFF)
+    set(${PROJECT_NAME}_IS_TOP_LEVEL OFF)
   endif()
-  unset(parent_directory)
 endif()
 
 # The library version is based on libtool versioning of the ABI. The set of