[CMake] Implement support for building the .NET bindings.

When using Mono support for installing/uninstalling the bindings
is also implemented. For Windows install/uninstall is not implemented
because the python build system does not implement it and Microsoft's
documentation (https://msdn.microsoft.com/en-us/library/dkkx7f79.aspx)
says that the gacutil should only be used for development and not for
production.

For now a warning is just emitted if ``INSTALL_DOTNET_BINDINGS``
is enabled and the .NET toolchain is native Windows. Someone with
better knowledge of how to correctly install assemblies under Windows
should implement this or remove this message.

A notable difference from the Python build system is the
``/linkresource:`` flag is not passed to the C# compiler. This means
a user of the .NET bindings will have to copy the Z3 library (i.e.
``libz3.dll``) to their application directory manually. The reason
for this difference is that using this flag requires the working
directory to be the directory containing the Z3 library (i.e.
``libz3.dll``) but setting this up with multi-configuration generators
doesn't currently seem possible.

.gitignore

@@ -84,3 +84,5 @@ src/CMakeLists.txt
 src/*/CMakeLists.txt
 src/*/*/CMakeLists.txt
 src/*/*/*/CMakeLists.txt
+src/api/dotnet/cmake_install_gac.cmake.in
+src/api/dotnet/cmake_uninstall_gac.cmake.in