[cmake] Fix builds with LLVM_ENABLE_PIC=0

[labath]

Summary

When this flag is specified, the target llvm-lto is not built, but is still
used as a dependency of the test targets. cmake 2.8 silently ignored this
situation, but with cmake_minimum_required(3.4) it becomes an error. Fix this
by avoiding the inclusion of the target as a dependency.

[beanz]

LGTM.

llvm/trunk/test/CMakeLists.txt

set(LLVM_TEST_DEPENDS
           llvm-diff
           llvm-dis
           llvm-dsymutil
           llvm-dwarfdump
           llvm-dwp
           llvm-extract
           llvm-lib
           llvm-link
-          llvm-lto
           llvm-mc
           llvm-mcmarkup
           llvm-nm
           llvm-objdump
           llvm-pdbdump
           llvm-profdata
           llvm-ranlib
           llvm-readobj
           llvm-rtdyld
           llvm-size
           llvm-split
           llvm-symbolizer
           llvm-tblgen
           not
           obj2yaml
           opt
           sancov
           sanstats
           verify-uselistorder
           yaml-bench
           yaml2obj
         )
 
+if(TARGET llvm-lto)
+  set(LLVM_TEST_DEPENDS ${LLVM_TEST_DEPENDS} llvm-lto)
+endif()
+
 # If Intel JIT events are supported, depend on a tool that tests the listener.
 if( LLVM_USE_INTEL_JITEVENTS )
   set(LLVM_TEST_DEPENDS ${LLVM_TEST_DEPENDS} llvm-jitlistener)
 endif( LLVM_USE_INTEL_JITEVENTS )
 
 if(TARGET LLVMgold)
   set(LLVM_TEST_DEPENDS ${LLVM_TEST_DEPENDS} LLVMgold)
 endif()
 
 if(TARGET llvm-go)
   set(LLVM_TEST_DEPENDS ${LLVM_TEST_DEPENDS} llvm-go)
 endif()
 
-if(APPLE)
+if(TARGET LTO)
   set(LLVM_TEST_DEPENDS ${LLVM_TEST_DEPENDS} LTO)
 endif()
 
 if(TARGET ocaml_llvm)
   set(LLVM_TEST_DEPENDS ${LLVM_TEST_DEPENDS}
           ocaml_llvm
           ocaml_llvm_all_backends
           ocaml_llvm_analysis