Export C++ API symbols from DLL on Windows

[jschueller]

The shared library symbols are not exported on windows.
Use a new COACD_API macro that expands to dllimport / dllexport directives.
CMakeLists has to be modified a bit to build static and shared separately instead of linking one into the other.
Avoids undefined references on windows when linking to the shared library _coacd:

/usr/x86_64-w64-mingw32/bin/ld: CMakeFiles/t_coacd.dir/objects.a(t_coacd.cxx.obj):t_coacd.cxx:(.text.startup+0x251)
: undefined reference to `coacd::CoACD(coacd::Mesh const&, double, int, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, int, int, int, int, int, bool, bool, bool, int, bool, double, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, unsigned int, bool)'
collect2: error: ld returned 1 exit status

[jschueller]

what do you say @SarahWeiii ?