hide all symbols in the shared library by default (if not exporting the private API) with GCC and Clang on non-Windows OSes (only API functions are exported in the Windows DLL)

this should remove undefined behavior e.g. when a user code compiled with different options than Gmsh also uses e.g. Eigen; in which case 2 versions of Eigen functions with exactly the same signature would exist, leading to really bad things (TM) happening