Silence some CMake 3.3 dev warnings in compiler-rt

Fix a trivial instance of CMP0054 that came up on llvmdev.

The other warnings were CMP0057, which is about using the same file as a
MAIN_DEPENDENCY multiple times.  The old behavior hasn't been a problem
yet, so I silenced the warning and filed PR23595 to document the issue
if someone cares.

llvm-svn: 237808
diff --git a/compiler-rt/cmake/Modules/CompilerRTUtils.cmake b/compiler-rt/cmake/Modules/CompilerRTUtils.cmake
index 0fdb784..f7f60a4 100644
--- a/compiler-rt/cmake/Modules/CompilerRTUtils.cmake
+++ b/compiler-rt/cmake/Modules/CompilerRTUtils.cmake
@@ -52,7 +52,7 @@
 
 macro(append_have_file_definition filename varname list)
   check_include_file("${filename}" "${varname}")
-  if (NOT "${varname}")
+  if (NOT ${varname})
     set("${varname}" 0)
   endif()
   list(APPEND ${list} "${varname}=${${varname}}")