cmake unity build

* Improve compile times, tested no ccache full compile:
    * Linux debug 9800x3d: 1m25s to 25s (3.4x speedup)
    * Linux release 9800x3d: ??? to 1m31s (???x speedup)
    * Windows debug 9800x3d: 1m21s to 13s (6.2x speedup)
    * Windows release 9800x3d: 1m24s to 18s (4.6x speedup)
    * Linux debug github runner: 11m50s to ???s (???x speedup)
    * Linux release github runner: 13m43s to ??? (???x speedup)
* From ~1.4k obj to ~190
* Lots of files not unity build due to IInput vs vgui::IInput
  ambiguity
* Not doing materialsystem as it .inc relies on 1-file-per-obj
* Windows cmd to measure:
    * powershell -Command "Measure-Command {cmake --build --preset windows-[debug/release] | Out-Default}"

* 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