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/lib/Makefile b/lib/Makefile
index 76731ab..6e0b014 100644
--- a/lib/Makefile
+++ b/lib/Makefile
@@ -98,8 +98,8 @@
 	@echo Cleaning library 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))
 
 libzstd.pc:
 libzstd.pc: libzstd.pc.in