cmake: add fallback method to finding zlib

CMakeLists.txt

@@ -101,6 +101,7 @@ check_include_files(openssl/opensslv.h HAVE_OPENSSLV_H)
 check_include_files(signal.h HAVE_SIGNAL_H)
 check_include_files(sys/random.h HAVE_SYS_RANDOM_H)
 check_include_files(unistd.h HAVE_UNISTD_H)
+check_include_files(zlib.h HAVE_ZLIB_H)
 
 check_symbol_exists(arc4random_buf stdlib.h HAVE_ARC4RANDOM_BUF)
 check_symbol_exists(asprintf stdio.h HAVE_ASPRINTF)
@@ -230,7 +231,7 @@ else()
 	if(NOT CRYPTO_FOUND AND NOT HAVE_OPENSSLV_H)
 		message(FATAL_ERROR "could not find libcrypto")
 	endif()
-	if(NOT ZLIB_FOUND)
+	if(NOT ZLIB_FOUND AND NOT HAVE_ZLIB_H)
 		message(FATAL_ERROR "could not find zlib")
 	endif()
 
@@ -240,6 +241,9 @@ else()
 	if(NOT CRYPTO_LIBRARIES)
 		set(CRYPTO_LIBRARIES "crypto")
 	endif()
+	if(NOT ZLIB_LIBRARIES)
+		set(ZLIB_LIBRARIES "z")
+	endif()
 
 	if(CMAKE_SYSTEM_NAME STREQUAL "Linux")
 		pkg_search_module(UDEV libudev REQUIRED)