exports: first try at error handling if a target is missing

Now, if an imported target depends on a library which must come
from some other export set, cmake generates a check which errors
out if that target does not exist. I guess instead of completely
erroring out it would be better to only make the find_package() fail.

Alex

Source/cmExportBuildFileGenerator.cxx

@@ -72,8 +72,9 @@ cmExportBuildFileGenerator
     if(!properties.empty())
       {
       // Get the rest of the target details.
+      std::vector<std::string> missingTargets;
       this->SetImportDetailProperties(config, suffix,
-                                      target, properties);
+                                      target, properties, missingTargets);
 
       // TOOD: PUBLIC_HEADER_LOCATION
       // This should wait until the build feature propagation stuff
@@ -82,6 +83,7 @@ cmExportBuildFileGenerator
       //                              properties);
 
       // Generate code in the export file.
+      this->GenerateMissingTargetsCheckCode(os, missingTargets);
       this->GenerateImportPropertyCode(os, config, target, properties);
       }
     }