Fix rpath-link for shared lib with only private deps

Under CMP0022 NEW behavior, the link interface is explicit even if there
are no public dependencies (`INTERFACE_LINK_LIBRARIES` is not set).
Mark it as such to activate our tracking of private runtime dependencies
of shared libraries for generation of `-rpath-link` flags.

Fixes: #19556

Source/cmGeneratorTarget.cxx

@@ -5660,7 +5660,7 @@ void cmGeneratorTarget::ComputeLinkInterfaceLibraries(
     return;
   }
   iface.Exists = true;
-  iface.Explicit = explicitLibraries != nullptr;
+  iface.Explicit = cmp0022NEW || explicitLibraries != nullptr;
 
   if (explicitLibraries) {
     // The interface libraries have been explicitly set.

Tests/RuntimePath/CMakeLists.txt

@@ -1,4 +1,4 @@
-cmake_minimum_required (VERSION 2.6)
+cmake_minimum_required (VERSION 3.15)
 project(RuntimePath C)
 
 # Add a simple chain of shared libraries that must be found.
@@ -31,3 +31,14 @@ if(CMAKE_SHARED_LIBRARY_SONAME_C_FLAG)
   set_property(TARGET bar2 PROPERTY LIBRARY_OUTPUT_DIRECTORY A)
   target_link_libraries(bar2 foo2)
 endif()
+
+# Add a library that is missing the rpath for its dependency.
+add_library(bar1_no_rpath SHARED bar1.c)
+set_property(TARGET bar1_no_rpath PROPERTY LIBRARY_OUTPUT_DIRECTORY B)
+set_property(TARGET bar1_no_rpath PROPERTY SKIP_BUILD_RPATH 1)
+target_link_libraries(bar1_no_rpath PRIVATE foo1)
+
+# Add an executable linking to the library with a missing dependency rpath.
+# CMake should generate the proper rpath-link flag to find it at build time.
+add_executable(main_with_bar1_no_rpath main.c)
+target_link_libraries(main_with_bar1_no_rpath bar1_no_rpath)