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

1. 
   hebasto commented at 11:25 am on June 19, 2025: member

   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:

   0$ cmake -B build -DBUILD_KERNEL_LIB=ON
   

   According to CMake’s documentation, this should configure libbitcoinkernel as STATIC. However, that’s not the case:

   0$ cmake --build build -t libbitcoinkernel
   1[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. Uses the SECP256K1_DISABLE_SHARED cache variable only when libsecp256k1 is included as a subproject.

   ---

   Alternatively, the entire code block can be dropped. In that case:

   1. The CMake-base build system will build a static library by default.
   2. During integration into a parent project, the static library can be forced as demonstrated here.
2. 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.

   533aea12f1
3. 
   hebasto commented at 11:25 am on June 19, 2025: member

   cc @theuni
4. real-or-random added the label build on Jun 19, 2025
5. real-or-random added the label refactor/smell on Jun 19, 2025

Contributors
hebasto

Labels
build refactor/smell