Set minimal zlib version to 1.2.3.

Rationale: 1.2.3 was a security update, and we should avoid using
versions with known security vulnerabilities.

cmake/OpenCVFindLibsGrfmt.cmake

@@ -6,7 +6,7 @@
 if(BUILD_ZLIB)
   ocv_clear_vars(ZLIB_FOUND)
 else()
-  include(FindZLIB)
+  find_package(ZLIB "${MIN_VER_ZLIB}")
   if(ZLIB_FOUND AND ANDROID)
     if(ZLIB_LIBRARIES STREQUAL "${ANDROID_SYSROOT}/usr/lib/libz.so")
       set(ZLIB_LIBRARIES z)

cmake/OpenCVMinDepVersions.cmake

 set(MIN_VER_CMAKE 2.8.7)
+set(MIN_VER_ZLIB 1.2.3)