MDEV-4607 : libreadline-related compilation problems on NetBSD.

Problem : 
libreadline.so was already present on the machine, however the cmake check NEW_READLINE_INTERFACE was unsuccessfull indicating, thus bundled library had to be used instead of  system library.
The problem was that the value for HAVE_HIST_ENTRY cmake variable  was cached with incorrect  value (1 on NetBSD).

The fix is to change HAVE_HIST_ENTRY to 0 with  CACHE FORCE, after switching to bundled readline.

cmake/readline.cmake

@@ -115,7 +115,7 @@ ENDMACRO()
 
 MACRO (MYSQL_USE_BUNDLED_READLINE)
   SET(USE_NEW_READLINE_INTERFACE 1)
-  SET(HAVE_HIST_ENTRY)
+  SET(HAVE_HIST_ENTRY 0 CACHE INTERNAL "" FORCE)
   SET(READLINE_INCLUDE_DIR ${CMAKE_SOURCE_DIR}/cmd-line-utils)
   SET(READLINE_LIBRARY readline)
   ADD_SUBDIRECTORY(${CMAKE_SOURCE_DIR}/cmd-line-utils/readline)