MEX library symbols visibility set to default

By default, the mexFunctions seems to be hidden in the newer Matlab version, this fix force the compiler to keep the symbols visible.

src/octave/CMakeLists.txt

@@ -6,6 +6,8 @@ if (Matlab_FOUND AND Matlab_MX_LIBRARY)
   cmake_minimum_required (VERSION 3.3) # for the matlab_add_mex macro
 
   matlab_add_mex (NAME nlopt_optimize-mex SRC nlopt_optimize-mex.c OUTPUT_NAME nlopt_optimize LINK_TO ${nlopt_lib})
+  set_target_properties(nlopt_optimize-mex PROPERTIES COMPILE_FLAGS "-fvisibility=default")
+  
   if (NLOPT_CXX)
     set_target_properties (nlopt_optimize-mex PROPERTIES LINKER_LANGUAGE CXX)
   endif ()