MDEV-5182 - build of 10.0.4/r3863 fails @ 'cmake' with

            -DINSTALL_SYSCONFDIR/-DDEFAULT_SYSCONFDIR specified

The reason for a bug is in support-files/CMakeLists.txt that tries to
install files in INSTALL_SYSCONF2DIR, without checking whether this
variable is actually defined. 

Normally INSTALL_SYSCONF2DIR is always defined if INSTALL_SYSCONFDIR is
defined, and support-files/CMakeLists.txt assumes that. But when one
specifies INSTALL_SYSCONFDIR manually on the command line, this
assumption might be broken.

support-files/CMakeLists.txt

@@ -105,12 +105,15 @@ IF(UNIX)
 
     INSTALL(FILES rpm/my.cnf DESTINATION ${INSTALL_SYSCONFDIR}
             COMPONENT Common)
+
+    IF (INSTALL_SYSCONF2DIR)
       INSTALL(FILES rpm/client.cnf DESTINATION ${INSTALL_SYSCONF2DIR}
               COMPONENT SharedLibraries)
       INSTALL(FILES rpm/mysql-clients.cnf DESTINATION ${INSTALL_SYSCONF2DIR}
               COMPONENT Client)
       INSTALL(FILES rpm/server.cnf DESTINATION ${INSTALL_SYSCONF2DIR}
               COMPONENT IniFiles)
+    ENDIF()
 
     # This is for SuSE:
     INSTALL(CODE "EXECUTE_PROCESS(COMMAND ${CMAKE_COMMAND} -E create_symlink