Do not export INTERFACE_LINK_LIBRARIES from non-linkable targets

When cmExportFileGenerator::SetImportLinkInterface exports the old
LINK_INTERFACE_LIBRARIES property values it skips doing so for
non-linkable targets because target->GetLinkInterface returns NULL for
such targets.

Since cmExportFileGenerator::PopulateInterfaceLinkLibrariesProperty
looks at the INTERFACE_LINK_LIBRARIES property directly instead of using
the computed link interface, teach it to skip exporting the property if
target->IsLinkable returns false.

Extend the RunCMake.CMP0022 test with a case covering this.  Simply
export an executable target that links to a library that is not
exported.

Source/cmExportFileGenerator.cxx

@@ -185,6 +185,10 @@ bool cmExportFileGenerator::PopulateInterfaceLinkLibrariesProperty(
                       ImportPropertyMap &properties,
                       std::vector<std::string> &missingTargets)
 {
+  if(!target->IsLinkable())
+    {
+    return false;
+    }
   const char *input = target->GetProperty("INTERFACE_LINK_LIBRARIES");
   if (input)
     {

Tests/RunCMake/CMP0022/CMP0022-export-exe-stderr.txt

@@ -0,0 +1 @@
+^$

Tests/RunCMake/CMP0022/CMP0022-export-exe.cmake

@@ -0,0 +1,9 @@
+enable_language(CXX)
+
+cmake_policy(SET CMP0022 NEW)
+
+add_library(testLib empty_vs6_1.cpp)
+add_executable(testExe empty_vs6_2.cpp)
+target_link_libraries(testExe testLib)
+
+export(TARGETS testExe FILE "${CMAKE_CURRENT_BINARY_DIR}/cmp0022NEW-exe.cmake")

Tests/RunCMake/CMP0022/RunCMakeTest.cmake

@@ -8,4 +8,5 @@ run_cmake(CMP0022-NOWARN-shared)
 run_cmake(CMP0022-NOWARN-static)
 run_cmake(CMP0022-NOWARN-static-link_libraries)
 run_cmake(CMP0022-export)
+run_cmake(CMP0022-export-exe)
 run_cmake(CMP0022-install-export)