Merge #1688: cmake: Avoid contaminating parent project's cache with BUILD_SHARED_LIBS

7b07b22 cmake: Avoid contaminating parent project's cache with BUILD_SHARED_LIBS (Hennadii Stepanov)

Pull request description:

  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.

  Consider configuring Bitcoin Core without explicit setting `BUILD_SHARED_LIBS`:
  ```
  $ cmake -B build -DBUILD_KERNEL_LIB=ON
  ```

  According to CMake’s documentation, this should configure `libbitcoinkernel` as `STATIC`.
  However, that's not the case:
  ```
  $ cmake --build build -t libbitcoinkernel
  [143/143] Linking CXX shared library lib/libbitcoinkernel.so
  ```

  This PR:
  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.  During integration into a parent project, the static library can be forced as demonstrated [here](bitcoin-core/minisketch#75 (comment)).

ACKs for top commit:
  purpleKarrot:
    ACK 7b07b22
  theuni:
    utACK 7b07b22

Tree-SHA512: 399a02e86093656ce70d9a0292a1f30834bcc5b9cf0f77d6465adc5e8a4d8e779684adedc40942eb931fed857399e4176a23fdbdf45f277184b28159fbc932d2

Author: real-or-random

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})