Match CMAKE_GENERATOR_TOOLSET variable case-insensitive

Match CMAKE_GENERATOR_TOOLSET variable case-insensitive

The Visual Studio CMake generator allows to select different toolsets.
One of these toolsets is Clang-Cl.

However, the generator does accept the toolset name case-agnostic, so it
could be "ClangCl", but also "Clangcl" or "clangcl" or ...
This value will be stored verbatim in variable CMAKE_GENERATOR_TOOLSET
by CMake. Therefore, this variable must be matched case-insensitive,
which is what this commit does.

fixes: #1576

Signed-off-by: Deniz Bahadir <deniz@code.bahadir.email>