fix: add check to avoid conflicts

seeing some build failure with stdlib_pch duplicates

  -- Precompiled headers were enabled
  CMake Error at build-support/macros-and-definitions.cmake:1272 (add_library):
    add_library cannot create ALIAS target "stdlib_pch" because another target
    with the same name already exists.
  Call Stack (most recent call first):
    CMakeLists.txt:149 (process_options)

https://github.com/Homebrew/homebrew-core/actions/runs/9240468411/job/25420819934?pr=172854#step:3:400

Signed-off-by: Rui Chen rui@chenrui.dev