Enable build on FreeBSD ports (includes DragonFly BSD)

Zstd has been introduced to FreeBSD ports
(http://www.freshports.org/archivers/zstd/) which DragonFly BSD also
uses.  FreeBSD and DragonFly use the install targets (albeit modified in
some cases) so they must be added to the associated Makefile filters.
diff --git a/Makefile b/Makefile
index 9f5e1eb..c7e7983 100644
--- a/Makefile
+++ b/Makefile
@@ -70,10 +70,10 @@
 	@echo Cleaning completed
 
 
-#------------------------------------------------------------------------
-#make install is validated only for Linux, OSX, kFreeBSD and Hurd targets
-#------------------------------------------------------------------------
-ifneq (,$(filter $(shell uname),Linux Darwin GNU/kFreeBSD GNU))
+#----------------------------------------------------------------------------------
+#make install is validated only for Linux, OSX, kFreeBSD, Hurd and some BSD targets
+#----------------------------------------------------------------------------------
+ifneq (,$(filter $(shell uname),Linux Darwin GNU/kFreeBSD GNU FreeBSD DragonFly))
 HOST_OS = POSIX
 install:
 	$(MAKE) -C $(ZSTDDIR) $@