Compile the C shared library with `-fvisibility=hidden -fvisibility-inlines-hidden`

[MLopez-Ibanez]

See https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#index-visibility-function-attribute
and https://gcc.gnu.org/wiki/Visibility

The starting point is https://github.com/multi-objective/moocore/blob/main/c/libmoocore-config.h

[MLopez-Ibanez]

Python is now compiled with -fvisibility=hidden (commit f83b871), which reduces the size of the shared library in Linux by around 15%.

[MLopez-Ibanez]

R is also compiled with -fvisibility=hidden but the reduction is only 4K. Perhaps it will make more of a difference when the library grows.