cmake now fails if zlib is not found

src/ngraph/CMakeLists.txt

@@ -100,6 +100,8 @@ include_directories(
     )
 
 if(LLVM_INCLUDE_DIR)
+    find_package(ZLIB REQUIRED)
+
     include_directories(SYSTEM ${LLVM_INCLUDE_DIR})
     link_directories(${LLVM_LIB_DIR})
     # Add sources for the CPU backend