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. Uses the `SECP256K1_DISABLE_SHARED` cache variable only when
   libsecp256k1 is included as a subproject.

Author: hebasto

CMakeLists.txt

@@ -33,10 +33,13 @@ 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(PROJECT_IS_TOP_LEVEL)
+  option(BUILD_SHARED_LIBS "Build shared libraries." ON)
+else()
+  option(SECP256K1_DISABLE_SHARED "Disable shared library. Overrides BUILD_SHARED_LIBS." OFF)
+  if(SECP256K1_DISABLE_SHARED)
+    set(BUILD_SHARED_LIBS OFF)
+  endif()
 endif()
 
 option(SECP256K1_INSTALL "Enable installation." ${PROJECT_IS_TOP_LEVEL})