<p>I did not have any issues using gcc 4.9.2 and MATLAB R2015a.  Can you try upgrading your compiler?  If that does not work, the issue lies with MATLAB, because the command that CMake is issuing to mex is valid (this is from your output above):</p>

<pre><code>/usr/local/MATLAB/R2015a/bin/mex -cxx -output ../../../../../matlab/gmm_mex.mexa64 CMakeFiles/gmm_mex.dir/gmm.o ../../../../../lib/libmlpack.so.1.0 /usr/lib/x86_64-linux-gnu/libxml2.so /usr/lib/libarmadillo.so /usr/lib/x86_64-linux-gnu/libboost_program_options.so /usr/lib/x86_64-linux-gnu/libboost_unit_test_framework.so /usr/lib/x86_64-linux-gnu/libboost_random.so /usr/lib/x86_64-linux-gnu/libxml2.so -lrt -Wl,-rpath,/home/User1/mlpack-1.0.12/build/lib
</code></pre>

<p>The same command is issued on my system, but it does not fail.</p>

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