Enable multithreading in lib build by default

lib/Makefile

@@ -32,6 +32,9 @@ LIBVER := $(shell echo $(LIBVER_SCRIPT))
 VERSION?= $(LIBVER)
 CCVER := $(shell $(CC) --version)
 
+# Multithreading can be forcibly disabled
+ZSTD_NO_MULTITHREAD ?= 0
+
 # ZSTD_LIB_MINIFY is a helper variable that
 # configures a bunch of other variables to space-optimized defaults.
 ZSTD_LIB_MINIFY ?= 0
@@ -292,16 +295,20 @@ libzstd : $(LIBZSTD)
 
 .PHONY: lib
 lib : libzstd.a libzstd
+	@if [ $(ZSTD_NO_MULTITHREAD) = 1 ]; then \
+		echo "multithreading disabled"; \
+	else \
+		echo "multithreading enabled"; \
+	fi
+
+# multithreading enabled by default unless otherwise specified
+ifneq ($(ZSTD_NO_MULTITHREAD), 1)
+  CPPFLAGS += -DZSTD_MULTITHREAD
+  LDFLAGS  += -pthread
+endif
 
-
-# note : do not define lib-mt or lib-release as .PHONY
+# note : do not define lib-release as .PHONY
 # make does not consider implicit pattern rule for .PHONY target
-
-%-mt : CPPFLAGS += -DZSTD_MULTITHREAD
-%-mt : LDFLAGS  += -pthread
-%-mt : %
-	@echo multi-threading build completed
-
 %-release : DEBUGFLAGS :=
 %-release : %
 	@echo release build completed
@@ -332,11 +339,13 @@ include $(wildcard $(DEPFILES))
 # Special case : building library in single-thread mode _and_ without zstdmt_compress.c
 ZSTDMT_FILES = compress/zstdmt_compress.c
 ZSTD_NOMT_FILES = $(filter-out $(ZSTDMT_FILES),$(ZSTD_FILES))
+ZSTD_NOMT_FLAGS = $(filter-out -DZSTD_MULTITHREAD,$(FLAGS))
+ZSTD_NOMT_LDFLAGS = $(filter-out -pthread,$(LDFLAGS))
 libzstd-nomt: LDFLAGS += -shared -fPIC -fvisibility=hidden
 libzstd-nomt: $(ZSTD_NOMT_FILES)
 	@echo compiling single-thread dynamic library $(LIBVER)
 	@echo files : $(ZSTD_NOMT_FILES)
-	$(CC) $(FLAGS) $^ $(LDFLAGS) $(SONAME_FLAGS) -o $@
+	$(CC) $(ZSTD_NOMT_FLAGS) $^ $(ZSTD_NOMT_LDFLAGS) $(SONAME_FLAGS) -o $@
 
 .PHONY: clean
 clean: