Install man page in /usr/share/man by default on OSX

Other packages apparently do this, and /usr/local/man isn't
in the default OSX man path.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
diff --git a/Makefile b/Makefile
index 6a3f85b..8d8802d 100644
--- a/Makefile
+++ b/Makefile
@@ -84,7 +84,12 @@
 INSTALL = install
 prefix = /usr/local
 bindir = $(prefix)/bin
+
+ifeq ($(UNAME), Darwin)
+mandir = /usr/share/man
+else
 mandir = $(prefix)/man
+endif
 
 all: .depend $(PROGS) $(SCRIPTS)