Revert "Remove zlib"

This reverts commit d874a1fd14bdf3df8ee232f539ac613adaae648c.

Conflicts:
* CMakeLists.txt:
  * ENABLE_ZLIB_SUPPORT: there has been a change immediately after
    where it was removed. Just re-add what was removed.
* tests/CMakeLists.txt:
  * ENABLE_ZLIB_SUPPORT: there has been a change immediately after
    where it was removed. Just re-add what was removed.
diff --git a/tests/CMakeLists.txt b/tests/CMakeLists.txt
index a1194e5..ecc33ee 100644
--- a/tests/CMakeLists.txt
+++ b/tests/CMakeLists.txt
@@ -9,6 +9,10 @@
     set(MBEDTLS_DIR ${CMAKE_SOURCE_DIR})
 endif()
 
+if(ENABLE_ZLIB_SUPPORT)
+    set(libs ${libs} ${ZLIB_LIBRARIES})
+endif(ENABLE_ZLIB_SUPPORT)
+
 find_package(PythonInterp)
 if(NOT PYTHONINTERP_FOUND)
     message(FATAL_ERROR "Cannot build test suites without Python 2 or 3")