Fix default library filename generation in CMakeLists.txt

[jtcooper10]

The repository's CMakeLists.txt file was not reliably giving a library file of the name structure godot-cpp.<platform>.<build_type>.<system_bits>. Tested on Windows using MSVC and CMake v3.19, which gave me the mysterious file: godot-cpp.windows..32.lib, which is missing the build_type entirely and detected 32-bit despite being on a 64-bit machine.

While I'm unsure why CMAKE_SIZEOF_VOID_P was resolving as 4 on my 64-bit machine, I figured it would be nice to be able to explicitly specify the architecture like you can in the SConstruct file (with bits=64). Additionally, the statement CMAKE_BUILD_TYPE STREQUAL "" evaluates to false when CMAKE_BUILD_TYPE is not defined.

With both of these updates, you should be able to replicate (using an out-of-source build on Windows as an example):

mkdir build
# Default build: godot-cpp.windows.debug.32.lib
cmake build
# Explicitly set: godot-cpp.windows.release.64.lib
cmake -DCMAKE_BUILD_TYPE=Release -DBITS=64 build

[akien-mga]

Seems fine, but needs a rebase. I'll push one.

[akien-mga]

Thanks!

[akien-mga]

Cherry-picked for 3.5.