Add more guards around zlib-dependent code

llvm-svn: 180084
diff --git a/llvm/lib/Support/Compression.cpp b/llvm/lib/Support/Compression.cpp
index d0adf79..497b17e 100644
--- a/llvm/lib/Support/Compression.cpp
+++ b/llvm/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;