Fix compiler warnings caused by redefinition of MYSQL_DATADIR, SHAREDIR and PLUGINDIR

/home/jan/mysql/10.1-orig/include/my_config.h:619:0: warning: "SHAREDIR" redefined [enabled by default]

 #define SHAREDIR "/usr/local/mysql/share"

 ^

In file included from /home/jan/mysql/10.1-orig/sql/log.h:20:0,

                 from /home/jan/mysql/10.1-orig/storage/xtradb/row/row0merge.cc:26:

/home/jan/mysql/10.1-orig/sql/unireg.h:40:0: note: this is the location of the previous definition

 #define SHAREDIR "share/"

 ^

In file included from /home/jan/mysql/10.1-orig/include/my_global.h:75:0,

                 from /home/jan/mysql/10.1-orig/sql/sql_plugin.h:28,

                 from /home/jan/mysql/10.1-orig/sql/structs.h:23,

                 from /home/jan/mysql/10.1-orig/sql/unireg.h:169,

                 from /home/jan/mysql/10.1-orig/sql/log.h:20,

                 from /home/jan/mysql/10.1-orig/storage/xtradb/row/row0merge.cc:26:

/home/jan/mysql/10.1-orig/include/my_config.h:621:0: warning: "MYSQL_DATADIR" redefined [enabled by default]

 #define MYSQL_DATADIR "/usr/local/mysql/data"

 ^

In file included from /home/jan/mysql/10.1-orig/sql/log.h:20:0,

                 from /home/jan/mysql/10.1-orig/storage/xtradb/row/row0merge.cc:26:

/home/jan/mysql/10.1-orig/sql/unireg.h:37:0: note: this is the location of the previous definition

 #define MYSQL_DATADIR  "data/"

 ^

In file included from /home/jan/mysql/10.1-orig/include/my_global.h:75:0,

                 from /home/jan/mysql/10.1-orig/sql/sql_plugin.h:28,

                 from /home/jan/mysql/10.1-orig/sql/structs.h:23,

                 from /home/jan/mysql/10.1-orig/sql/unireg.h:169,

                 from /home/jan/mysql/10.1-orig/sql/log.h:20,

                 from /home/jan/mysql/10.1-orig/storage/xtradb/row/row0merge.cc:26:

/home/jan/mysql/10.1-orig/include/my_config.h:623:0: warning: "PLUGINDIR" redefined [enabled by default]

 #define PLUGINDIR "/usr/local/mysql/lib/plugin"

 ^

In file included from /home/jan/mysql/10.1-orig/sql/log.h:20:0,

                 from /home/jan/mysql/10.1-orig/storage/xtradb/row/row0merge.cc:26:

/home/jan/mysql/10.1-orig/sql/unireg.h:43:0: note: this is the location of the previous definition

 #define PLUGINDIR "lib/plugin"

 ^