cmComputeLinkInformation: Lookup a target only if we have one

The change in commit 27252b24 (cmComputeLinkInformation: Simplify
generator object access, 2015-08-02) broke the conditional use of a
target introduced in commit 41abdc17 (cmGeneratorTarget: Move GetSOName
from cmTarget, 2015-08-04).  Restore the conditional lookup.  Add a test
case that hacks platform information variables to trigger this code
everywhere.

Source/cmComputeLinkInformation.cxx

@@ -760,7 +760,7 @@ void cmComputeLinkInformation::AddSharedDepItem(std::string const& item,
     return;
     }
 
-  cmGeneratorTarget *gtgt = this->GlobalGenerator->GetGeneratorTarget(tgt);
+  cmGeneratorTarget *gtgt = 0;
 
   // Get a full path to the dependent shared library.
   // Add it to the runtime path computation so that the target being

Tests/RunCMake/target_link_libraries/RunCMakeTest.cmake

@@ -7,3 +7,4 @@ run_cmake(CMP0023-NEW-2)
 run_cmake(MixedSignature)
 run_cmake(Separate-PRIVATE-LINK_PRIVATE-uses)
 run_cmake(SubDirTarget)
+run_cmake(SharedDepNotTarget)

Tests/RunCMake/target_link_libraries/SharedDepNotTarget.cmake

@@ -0,0 +1,10 @@
+enable_language(C)
+set(CMAKE_LINK_DEPENDENT_LIBRARY_DIRS 1)
+set(CMAKE_SHARED_LIBRARY_SUFFIX ".so")
+add_library(imported SHARED IMPORTED)
+set_target_properties(imported PROPERTIES
+  IMPORTED_LOCATION "imported"
+  IMPORTED_LINK_DEPENDENT_LIBRARIES "/path/to/libSharedDep.so"
+  )
+add_executable(empty empty.c)
+target_link_libraries(empty imported)