try to keep libzstd.a "as is" once created

to be compatible with scenarios such as
`make -j allmost`

Makefile

@@ -48,7 +48,7 @@ allmost: allzstd zlibwrapper
 
 # skip zwrapper, can't build that on alternate architectures without the proper zlib installed
 .PHONY: allzstd
-allzstd: lib-all
+allzstd: lib
 	$(Q)$(MAKE) -C $(PRGDIR) all
 	$(Q)$(MAKE) -C $(TESTDIR) all
 
@@ -57,9 +57,8 @@ all32:
 	$(MAKE) -C $(PRGDIR) zstd32
 	$(MAKE) -C $(TESTDIR) all32
 
-.PHONY: lib lib-release libzstd.a
-lib-all : lib
-lib lib-release lib-all :
+.PHONY: lib lib-release
+lib lib-release :
 	$(Q)$(MAKE) -C $(ZSTDDIR) $@
 
 .PHONY: zstd zstd-release