SCons: unify code generations routine and minimize timestamp changes

Previously, all of the code generation routines would just needlessly
write the same files over and over, even when not needed.

This became a problem with the advent of the experimental ninja backend
for SCons, which can be trivially enabled with a few lines of code and
relies on timestamp changes, making it thus impractical.

platform/SCsub

@@ -1,5 +1,7 @@
 #!/usr/bin/env python
 
+import methods
+
 Import("env")
 
 env.platform_sources = []
@@ -18,12 +20,7 @@ reg_apis_inc += "\n"
 reg_apis += "}\n\n"
 unreg_apis += "}\n"
 
-# NOTE: It is safe to generate this file here, since this is still execute serially
-with open("register_platform_apis.gen.cpp", "w", encoding="utf-8", newline="\n") as f:
-    f.write(reg_apis_inc)
-    f.write(reg_apis)
-    f.write(unreg_apis)
-
+methods.write_file_if_needed("register_platform_apis.gen.cpp", reg_apis_inc + reg_apis + unreg_apis)
 env.add_source_files(env.platform_sources, "register_platform_apis.gen.cpp")
 
 lib = env.add_library("platform", env.platform_sources)