Drop bundled zlib

We're not doing a sufficient job of keeping this updated so list
it as an external requirement instead.
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 882077a..f732f2f 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -116,14 +116,7 @@
 endif()
 
 # Check for zlib
-find_package(ZLIB)
-option(USE_INCLUDED_ZLIB "Force use of the bundled zlib")
-if(NOT ZLIB_FOUND)
-  set(USE_INCLUDED_ZLIB 1)
-endif()
-if(USE_INCLUDED_ZLIB)
-  message(STATUS "Using included zlib library")
-endif()
+find_package(ZLIB REQUIRED)
 
 # Check for gettext
 option(ENABLE_NLS "Enable translation of program messages" ON)