fix education decoder test

when `zstd` in not installed on local system
by allowing `ZSTD` variable to hold a custom location for the binary

Makefile

@@ -69,7 +69,7 @@ test: MOREFLAGS += -g -DDEBUGLEVEL=$(DEBUGLEVEL) -Werror
 test:
 	MOREFLAGS="$(MOREFLAGS)" $(MAKE) -j -C $(PRGDIR) allVariants
 	$(MAKE) -C $(TESTDIR) $@
-	$(MAKE) -C doc/educational_decoder test
+	ZSTD=../../programs/zstd $(MAKE) -C doc/educational_decoder test
 
 ## shortest: same as `make check`
 .PHONY: shortest

doc/educational_decoder/Makefile

@@ -1,10 +1,21 @@
+# ################################################################
+# Copyright (c) 2016-present, Yann Collet, Facebook, Inc.
+# All rights reserved.
+#
+# This source code is licensed under both the BSD-style license (found in the
+# LICENSE file in the root directory of this source tree) and the GPLv2 (found
+# in the COPYING file in the root directory of this source tree).
+# ################################################################
+
+ZSTD ?= zstd   # requires zstd installation on local system
+DIFF ?= diff
 HARNESS_FILES=*.c
 
 MULTITHREAD_LDFLAGS = -pthread
 DEBUGFLAGS= -g -DZSTD_DEBUG=1
 CPPFLAGS += -I$(ZSTDDIR) -I$(ZSTDDIR)/common -I$(ZSTDDIR)/compress \
             -I$(ZSTDDIR)/dictBuilder -I$(ZSTDDIR)/deprecated -I$(PRGDIR)
-CFLAGS   ?= -O3
+CFLAGS   ?= -O2
 CFLAGS   += -Wall -Wextra -Wcast-qual -Wcast-align -Wshadow                 \
             -Wstrict-aliasing=1 -Wswitch-enum                               \
             -Wredundant-decls -Wstrict-prototypes -Wundef                   \
@@ -22,16 +33,22 @@ clean:
 	@$(RM) -rf harness.dSYM
 
 test: harness
-	@zstd README.md -o tmp.zst
+	#
+	# Testing single-file decompression with educational decoder
+	#
+	@$(ZSTD) README.md -o tmp.zst
 	@./harness tmp.zst tmp
-	@diff -s tmp README.md
+	@$(DIFF) -s tmp README.md
 	@$(RM) -f tmp*
-	# present files for training multiple times, to reach minimum threshold
-	@zstd --train harness.c zstd_decompress.c zstd_decompress.h README.md \
+	#
+	# Testing dictionary decompression with education decoder
+	#
+	# note : files are presented multiple for training, to reach minimum threshold
+	@$(ZSTD) --train harness.c zstd_decompress.c zstd_decompress.h README.md \
                   harness.c zstd_decompress.c zstd_decompress.h README.md \
                   harness.c zstd_decompress.c zstd_decompress.h README.md
-	@zstd -D dictionary README.md -o tmp.zst
+	@$(ZSTD) -D dictionary README.md -o tmp.zst
 	@./harness tmp.zst tmp dictionary
-	@diff -s tmp README.md
+	@$(DIFF) -s tmp README.md
 	@$(RM) -f tmp* dictionary
-	@make clean
+	@$(MAKE) clean