cmake: Cleanup (#2535)

- Don't call enable_language before project.
See CMake docs for CMP0165.
- Remove boost-related cache variables.
It's too late to set them because Boost::headers has already been found.
- Don't build SDL3-test library.
Not used by shadps4.
- Remove /Zc:preprocessor from toml11's INTERFACE_COMPILE_OPTIONS.
Clang-cl does not support this flag and emits a lot of warnings.
- Remove dummy.cpp and make gcn a proper INTERFACE target.

externals/gcn/CMakeLists.txt

@@ -3,6 +3,10 @@
 
 project(gcn LANGUAGES CXX)
 
-add_library(gcn dummy.cpp)
+add_library(gcn INTERFACE)
+target_sources(gcn PRIVATE
+  "include/gcn/si_ci_vi_merged_offset.h"
+  "include/gcn/si_ci_vi_merged_pm4_it_opcodes.h"
+)
 
 target_include_directories(gcn INTERFACE include)

externals/gcn/dummy.cpp

@@ -1,2 +0,0 @@
-//  SPDX-FileCopyrightText: Copyright 2024 shadPS4 Emulator Project
-//  SPDX-License-Identifier: GPL-2.0-or-later