[make] better discovery of `-s` make flag

Summary:
Single-letter options that end up in `MAKEFLAGS` seem to be always the first
word of `$MAKEFLAGS`. We can use this instead of dodgy filtering.

Reviewed By: jeremydubreil

Differential Revision: D9149899

fbshipit-source-id: a621c3137

Makefile.config

@@ -109,8 +109,7 @@ define copy_or_same_file
 endef
 
 INTERACTIVE = $(shell [ -t 0 ] && echo 1)
-# remove "jobserver-fds" because it contains "s"...
-SILENT = $(findstring s,$(subst jobserver-fds,,$(MAKEFLAGS)))
+SILENT = $(findstring s,$(firstword $(MAKEFLAGS)))
 
 ifeq (1,$(INTERACTIVE))
 TERM_ERROR = $(shell printf '\e[31;1m')