hide all symbols in the shared library by default (if not exporting the...

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