CMake unity build (#1982)

* Improve compile times, no ccache full compile (these are just sample times to give a rough idea of the speedups, it can varies):
    * Linux debug 9800x3d: 1m13s to 23s (3.2x speedup)
    * Linux release 9800x3d: 1m36s to 1m21s (1.2x speedup)
    * Windows debug 9800x3d: 1m21s to 14s (4.7x speedup)
    * Windows release 9800x3d: 1m24s to 18s (4.6x speedup)
    * Time VS in CMake library build stage: https://github.com/NeotokyoRebuild/neo/actions/runs/27134842462 (non-unity) VS https://github.com/NeotokyoRebuild/neo/actions/runs/27227387768 (unity)
    * Linux debug github runner: 12m16s to 2m6s (5.8x speedup) | Total down from 16m16s to 5m27s
    * Linux release github runner: 14m3s to 4m57s (2.8x speedup) | Total down from 18m16s to 9m41s
    * Windows debug github runner: 10m56s to 1m24s (7.8x speedup) | Total down from 14m17s to 4m58s
    * Windows release github runner: 11m22s to 2m38s (4.3x speedup) | Total down from 14m45s to   6m5s
* Lots of files not unity build due to IInput vs vgui::IInput ambiguity
* Some files left out cause easier to leave it out than dealing with it
* Not doing materialsystem as it .inc relies on 1-file-per-obj
* Few files left out of unity due to issues shown up in runtime
* Object/compilation units from 1.4k to ~166

* fixes #1981

Author: nullsystem

src/CMakeLists.txt

@@ -7,7 +7,8 @@ set(CMAKE_VERBOSE_MAKEFILE TRUE)
 message(STATUS "CMAKE_VERSION: ${CMAKE_VERSION}")
 
 string(TIMESTAMP BUILD_DATETIME UTC)
-message(STATUS "TIMESTAMP: ${BUILD_DATETIME}")
+# NEO TODO (nullsystem): This screwed up nvim's :make
+# message(STATUS "TIMESTAMP: ${BUILD_DATETIME}")
 
 list(APPEND CMAKE_MODULE_PATH "${CMAKE_SOURCE_DIR}/cmake")
 
@@ -179,6 +180,7 @@ if(OS_WINDOWS)
         /errorReport:prompt # ErrorReporting - Prompt Immediately
         /W4
         /Gw # Optimize Global Data
+		/bigobj # Required for unity build
 
         /wd4456
         /wd4457