use truncating division for code that explicitly needs an int

We are going to truncate it down below with int() anyway, no need to
carry extra precision. And mypy complains if the type changes between
float and int.

mesonbuild/backend/ninjabackend.py

@@ -114,7 +114,7 @@ def get_rsp_threshold():
         # single argument; see MAX_ARG_STRLEN
         limit = 131072
     # Be conservative
-    limit = limit / 2
+    limit = limit // 2
     return int(os.environ.get('MESON_RSP_THRESHOLD', limit))
 
 # a conservative estimate of the command-line length limit