Allow the parent CMake file to specify the zlib include dir.

Change-Id: I632d37adf810125a818a1aa4036062735f058c63
diff --git a/CMakeLists.txt b/CMakeLists.txt
index a938d1d..9873504 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -46,6 +46,7 @@
 include_directories("include")
 include_directories("lib")
 include_directories(BEFORE SYSTEM  ${OPENSSL_INCLUDE_DIR})
+include_directories(BEFORE SYSTEM  ${ZLIB_INCLUDE_DIR})
 include_directories(${CMAKE_CURRENT_BINARY_DIR})
 add_library(curl
   "lib/amigaos.c"