cmake: Avoid contaminating parent project's cache with BUILD_SHARED_LIBS

The CMake cache is global in scope. Therefore, setting the standard
cache variable `BUILD_SHARED_LIBS` can inadvertently affect the behavior
of a parent project.

This change:
1. Sets the `BUILD_SHARED_LIBS` cache variable only when libsecp256k1 is
   the top-level project.
2. Removes the `SECP256K1_DISABLE_SHARED` cache variable. This enables
   parent projects that include libsecp256k1 as a subproject to rely
   solely on standard CMake variables for configuring the library type.

Author: hebasto

CMakeLists.txt

@@ -34,10 +34,8 @@ set(CMAKE_C_EXTENSIONS OFF)
 #=============================
 # Configurable options
 #=============================
-option(BUILD_SHARED_LIBS "Build shared libraries." ON)
-option(SECP256K1_DISABLE_SHARED "Disable shared library. Overrides BUILD_SHARED_LIBS." OFF)
-if(SECP256K1_DISABLE_SHARED)
-  set(BUILD_SHARED_LIBS OFF)
+if(libsecp256k1_IS_TOP_LEVEL)
+  option(BUILD_SHARED_LIBS "Build shared libraries." ON)
 endif()
 
 option(SECP256K1_INSTALL "Enable installation." ${PROJECT_IS_TOP_LEVEL})