added target make check

according to GNU Makefile conventions,
the Makefile should feature a make check target
to self-test the generated program:
https://www.gnu.org/prep/standards/html_node/Standard-Targets.html .

this is much less thorough and less taxing than `make test`,
and can be run on any target in a reasonable timeframe (several seconds).

Makefile

diff --git a/Makefile b/Makefile
index 83320fa..8a7d155 100644
--- a/Makefile
+++ b/Makefile
@@ -1,10 +1,8 @@
 # ################################################################
 # LZ4 - Makefile
-# Copyright (C) Yann Collet 2011-2016
+# Copyright (C) Yann Collet 2011-present
 # All rights reserved.
 #
-# This Makefile is validated for Linux, macOS, *BSD, Hurd, Solaris, MSYS2 targets
-#
 # BSD license
 # Redistribution and use in source and binary forms, with or without modification,
 # are permitted provided that the following conditions are met:
@@ -122,6 +120,10 @@
 list:
 	@$(MAKE) -pRrq -f $(lastword $(MAKEFILE_LIST)) : 2>/dev/null | awk -v RS= -F: '/^# File/,/^# Finished Make data base/ {if ($$1 !~ "^[#.]") {print $$1}}' | sort | egrep -v -e '^[^[:alnum:]]' -e '^$@$$' | xargs
 
+.PHONY: check
+check:
+	$(MAKE) -C $(TESTDIR) test-lz4-essentials
+
 .PHONY: test
 test:
 	$(MAKE) -C $(TESTDIR) $@