CMake by default searches for shared libs before static libs. Sometimes a project needs to specifically link external or internal static libs, even if shared libs are present. A simple cross-platform way to do this is to set CMAKE_FIND_LIBRARY_SUFFIXES before any find_library() or find_package():

if(NOT BUILD_SHARED_LIBS)
  set(CMAKE_FIND_LIBRARY_SUFFIXES .a .lib)
endif()

This takes advantage of de facto static library extensions for macOS, Windows and Linux, using CMake system variables.