Fix Z3Config.cmake.in when generating a static library

[mrexodia]

If you use -DZ3_BUILD_LIBZ3_SHARED=OFF and then the following CMakeLists.txt to find the installed Z3:

cmake_minimum_required(VERSION 3.15)

project(z3_static_test)

find_package(Z3 REQUIRED CONFIG)

add_executable(z3_static_test src/main.cpp)

target_link_libraries(z3_static_test PRIVATE z3::libz3)

You will get the following error:

-- Configuring done
CMake Error at CMakeLists.txt:51 (add_executable):
  Target "z3_static_test" links to target "Threads::Threads" but the target
  was not found.  Perhaps a find_package() call is missing for an IMPORTED
  target, or an ALIAS target is missing?


-- Generating done
CMake Generate step failed.  Build files cannot be regenerated correctly.

This is because the find_dependency calls for Threads (and GMP) are missing. When building a shared library this is no problem because these are not part of the IMPORTED target, but when you build a static library these dependencies will be propagated to the final executable you link.