CMake: simplify build rules for compiler-rt unit tests. This fixes warnings in Ninja build tree.

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@173677 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/cmake/Modules/AddCompilerRT.cmake b/cmake/Modules/AddCompilerRT.cmake
index d084bf0..742d81f 100644
--- a/cmake/Modules/AddCompilerRT.cmake
+++ b/cmake/Modules/AddCompilerRT.cmake
@@ -114,12 +114,10 @@
 macro(add_compiler_rt_test test_suite test_name)
   parse_arguments(TEST "OBJECTS;DEPS;LINK_FLAGS" "" ${ARGN})
   set(output_bin "${CMAKE_CURRENT_BINARY_DIR}/${test_name}")
-  add_custom_command(
-    OUTPUT ${output_bin}
+  add_custom_target(${test_name}
     COMMAND clang ${TEST_OBJECTS} -o "${output_bin}"
             ${TEST_LINK_FLAGS}
     DEPENDS clang ${TEST_DEPS})
-  add_custom_target(${test_name} DEPENDS ${output_bin})
   # Make the test suite depend on the binary.
   add_dependencies(${test_suite} ${test_name})
 endmacro()