CMakePackage: Set policy CMP0042 NEW on macos (#46114)

so linking to shared libraries works well at runtime on darwin for all packages with cmake_minimum_required < 3.

lib/spack/spack/build_systems/cmake.py

@@ -108,6 +108,11 @@ def _conditional_cmake_defaults(pkg: spack.package_base.PackageBase, args: List[
     if _supports_compilation_databases(pkg):
         args.append(CMakeBuilder.define("CMAKE_EXPORT_COMPILE_COMMANDS", True))
 
+    # Enable MACOSX_RPATH by default when cmake_minimum_required < 3
+    # https://cmake.org/cmake/help/latest/policy/CMP0042.html
+    if pkg.spec.satisfies("platform=darwin") and cmake.satisfies("@3:"):
+        args.append(CMakeBuilder.define("CMAKE_POLICY_DEFAULT_CMP0042", "NEW"))
+
 
 def generator(*names: str, default: Optional[str] = None):
     """The build system generator to use.