MySQL Plugin failed to install due to “undefined symbol”

You didn't say, in your question what the "file not present" message was, but I'm guessing it's this:

ERROR 1126 (HY000) at line 29: Can't open shared library 'lib_mysqludf_sys.so' 
(errno: 0 /usr/local/mysql/lib/plugin/lib_mysqludf_sys.so: cannot open shared 
object file: No such file or directory)
ERROR: unable to install the UDF

That makes sense, because you're telling gcc to write the file to /usr/lib/lib_mysqludf_sys.so... that's the -o option in your command line. Give gcc the path MySQL is expecting and the rest of the installation should work.

gcc -m64 -fPIC -Wall -I/usr/include/mysql -I. -shared lib_mysqludf_sys.c \ 
-o /usr/local/mysql/lib/plugin/lib_mysqludf_sys.so \
-L/usr/lib/x86_64-linux-gnu/libstdc++.so.6

Also, whatever you're planning to do with this... don't say I didn't recommend against it. You're introducing both a potential security vulnerability and potentional performance and stability liability if these tools are deployed. This is not because there's anything wrong with the utility, but because of the amount of somewhat unorthodox functionality it opens up.

It's pretty cool, I admit. The lib_mysqludf_sys group of user-defined functions allow some interesting but easily-misappropriated capabilities, letting you spawn system commands and get either their generated output with sys_eval() or their return value with sys_exec() but really, I tend to suspect there's a reason why MySQL doesn't have these capabilities built in.

mysql> select sys_eval('df -k | grep xvda | tr -d "\n"') as cool_function_but_bad_idea;
+-----------------------------------------------------+
| cool_function_but_bad_idea                          |
+-----------------------------------------------------+
| /dev/xvda1       8256952   1216636   6620888  16% / |
+-----------------------------------------------------+
1 row in set (0.81 sec)

mysql> select sys_exec('/bin/false'), sys_exec('/bin/true');
+------------------------+-----------------------+
| sys_exec('/bin/false') | sys_exec('/bin/true') |
+------------------------+-----------------------+
|                    256 |                     0 |
+------------------------+-----------------------+
1 row in set (1.59 sec)

Update to address this error message:

gcc -Wall -m64 -I/usr/include/mysql -I. -shared lib_mysqludf_sys.c -o 
/usr/lib/lib_mysqludf_sys.so -fPIC gcc: error: lib_mysqludf_sys.c: No such 
file or directory gcc: fatal error: no input files compilation terminated

The statement in question is directing "gcc" to compile a 64-bit version from the source code, which is found in lib_mysqludf_sys.c. This is one of the files you downloaded, so, to compile it, you need to be inside the directory where you downloaded the lib_mysqludf_sys files. This error suggests that you aren't. The download package isn't limited to 32 bit, it's just that it only builds a 32 bit version unless you use the gcc -m64 -fPIC ... statement.

The right place to ask this question in by filing a ticket in the MariaDB Jira or on the #mariadb IRC channel on freenode.