always link C/C with external (to C/C) zlib

it'll be either system zlib.so or bundled with the server (but still
external to C/C) zlib.a

cmake/mariadb_connector_c.cmake

@@ -6,10 +6,7 @@ ENDIF()
 
 SET(CONC_WITH_SIGNCODE ${SIGNCODE})
 SET(SIGN_OPTIONS ${SIGNTOOL_PARAMETERS})
-
-IF(NOT TARGET zlib)
-  SET(CONC_WITH_EXTERNAL_ZLIB ON)
-ENDIF()
+SET(CONC_WITH_EXTERNAL_ZLIB ON)
 
 IF(SSL_DEFINES MATCHES "YASSL")
   IF(WIN32)