[cmake] refactor: Use CPM over submodules (#143)

Transfers the majority of submodules and large externals to CPM, using source archives rather than full Git clones. Not only does this save massive amounts of clone and configure time, but dependencies are grabbed on-demand rather than being required by default. Additionally, CPM will (generally) automatically search for system dependencies, though certain dependencies have options to control this.

Testing shows gains ranging from 5x to 10x in terms of overall clone/configure time.

Reviewed-on: #143
Reviewed-by: CamilleLaVey <camillelavey99@gmail.com>

externals/libusb/CMakeLists.txt

@@ -235,6 +235,12 @@ else() # MINGW OR (${CMAKE_SYSTEM_NAME} MATCHES "Linux")
         )
     endif()
 
+    if(UNIX)
+        set(PLATFORM_POSIX TRUE)
+    else()
+        set(PLATFORM_WINDOWS TRUE)
+    endif()
+
     include(CheckFunctionExists)
     include(CheckIncludeFiles)
     include(CheckTypeSize)