Add more guards around zlib-dependent code

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@180084 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Support/Compression.cpp b/lib/Support/Compression.cpp
index d0adf79..497b17e 100644
--- a/lib/Support/Compression.cpp
+++ b/lib/Support/Compression.cpp
@@ -23,7 +23,7 @@
 
 using namespace llvm;
 
-#if LLVM_ENABLE_ZLIB == 1
+#if LLVM_ENABLE_ZLIB == 1 && HAVE_LIBZ
 static int encodeZlibCompressionLevel(zlib::CompressionLevel Level) {
   switch (Level) {
     case zlib::NoCompression: return 0;