[make] do not take `--flags` into account when looking for `-s`

Summary:
In some `make` versions, when not passing any single-letter options explicitly
from the command line it could be that the first word of `MAKEFLAGS` is some
`--long-option` instead, and this trips the detections of `-s`. If the first
word starts with `-` it will now be ignored.

Reviewed By: ddino

Differential Revision: D9239290

fbshipit-source-id: c1120e055

Makefile.config

@@ -111,7 +111,7 @@ define copy_or_same_file
 endef
 
 INTERACTIVE = $(shell [ -t 0 ] && echo 1)
-SILENT = $(findstring s,$(firstword $(MAKEFLAGS)))
+SILENT = $(findstring s,$(filter-out -%,$(firstword $(MAKEFLAGS))))
 
 ifeq (1,$(INTERACTIVE))
 TERM_ERROR = $(shell printf '\e[31;1m')