As noted by Steffen Schmidt, in most projects the installation prefix is
spelled in caps, so follow this rule in osl.
Also use ?:= as the assignement operator as it creates a simply expanded
variable which is more suitable for the PREFIX variable.
# where to install
-prefix := /usr/local
-libdir := $(prefix)/lib
-includedir := $(prefix)/include
-bindir := $(prefix)/bin
-mandir := $(prefix)/man/man1
+PREFIX ?:= /usr/local
+libdir := $(PREFIX)/lib
+includedir := $(PREFIX)/include
+bindir := $(PREFIX)/bin
+mandir := $(PREFIX)/man/man1
objects := osl.o util.o rbtree.o sha1.o
fsck_objects := fsck.fsck.o osl.fsck.o util.fsck.o rbtree.fsck.o sha1.fsck.o fsck.cmdline.o