CMakeLists: Link in zlib privately where applicable

Also, use the target name introduced via find_package(ZLIB) instead
of hardcoding the 'z' name in the link libraries.

Externals/zlib/CMakeLists.txt

@@ -77,3 +77,4 @@ set(ZLIB_SRCS
 )
 
 add_library(z STATIC ${ZLIB_SRCS} ${ZLIB_DLL_SRCS} ${ZLIB_PUBLIC_HDRS} ${ZLIB_PRIVATE_HDRS})
+add_library(ZLIB::ZLIB ALIAS z)

Source/Core/Core/CMakeLists.txt

@@ -288,11 +288,11 @@ PUBLIC
   videonull
   videoogl
   videosoftware
-  z
 
 PRIVATE
   bdisasm
   ${LZO}
+  ZLIB::ZLIB
 )
 
 if (APPLE)

Source/Core/DiscIO/CMakeLists.txt

@@ -21,3 +21,8 @@ add_library(discio
   WiiSaveBanner.cpp
   WiiWad.cpp
 )
+
+target_link_libraries(discio
+PRIVATE
+  ZLIB::ZLIB
+)