Don't download anything.
Don't try to execute git.
Remove -Werror.

Index: CMakeLists.txt
--- CMakeLists.txt.orig
+++ CMakeLists.txt
@@ -160,12 +160,6 @@ if (MACOS_USE_DEPENDENCIES)
     endif ()
 endif ()
 
-# If OS is Linux, import OpenSoundEffects and OpenMusic
-if(UNIX AND NOT APPLE)
-    set(DOWNLOAD_OPENMSX ON)
-    set(DOWNLOAD_OPENSFX ON)
-endif()
-
 # LIST of supported flags, use SET_CHECK_CXX_FLAGS() to apply to target.
 # Use ADD_CHECK_CXX_COMPILER_FLAG() to add to list.
 set(SUPPORTED_CHECK_CXX_COMPILER_FLAGS "")
@@ -184,25 +178,6 @@ execute_process(
     ERROR_QUIET
 )
 
-# Define current git branch
-execute_process(
-    COMMAND git rev-parse --abbrev-ref HEAD
-    WORKING_DIRECTORY ${ROOT_DIR}
-    OUTPUT_VARIABLE OPENRCT2_BRANCH
-    OUTPUT_STRIP_TRAILING_WHITESPACE
-    ERROR_QUIET
-)
-
-# Define short commit hash
-execute_process(
-    COMMAND git rev-parse --short HEAD
-    WORKING_DIRECTORY ${ROOT_DIR}
-    OUTPUT_VARIABLE OPENRCT2_COMMIT_SHA1_SHORT
-    OUTPUT_STRIP_TRAILING_WHITESPACE
-    ERROR_QUIET
-)
-
-
 if (NOT DISABLE_DISCORD_RPC)
     if (UNIX AND NOT APPLE)
         find_package(DiscordRPC)
@@ -332,7 +307,7 @@ else ()
 
     # Compiler flags
     set(DEBUG_LEVEL 0 CACHE STRING "Select debug level for compilation. Use value in range 0–3.")
-    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -fstrict-aliasing -Werror -Wundef -Wmissing-declarations -Winit-self -Wall -Wextra -Wshadow")
+    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -fstrict-aliasing -Wundef -Wmissing-declarations -Winit-self -Wall -Wextra -Wshadow")
     set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wno-unknown-pragmas -Wno-missing-braces -Wno-comment -Wnonnull -Wno-unused-parameter -Wno-attributes")
     set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -DDEBUG=${DEBUG_LEVEL}")
 
