diff --git a/Makefile b/Makefile
index b2d1a32..3c3639f 100644
--- a/Makefile
+++ b/Makefile
@@ -3,22 +3,22 @@ VERSION = 0.4.11
 SHELL = /bin/sh
 top_srcdir = .
 srcdir = .
-prefix = /usr
-exec_prefix = ${prefix}
-
-bindir = ${exec_prefix}/bin
-sbindir = ${exec_prefix}/sbin
-libexecdir = ${exec_prefix}/libexec
-datadir = ${prefix}/share
-sysconfdir = ${prefix}/etc
-sharedstatedir = ${prefix}/com
-localstatedir = ${prefix}/var
-libdir = ${exec_prefix}/lib
-infodir = ${prefix}/info
+prefix ?= /usr
+exec_prefix ?= ${prefix}
+
+bindir ?= ${exec_prefix}/bin
+sbindir ?= ${exec_prefix}/sbin
+libexecdir ?= ${exec_prefix}/libexec
+datadir ?= ${prefix}/share
+sysconfdir ?= ${prefix}/etc
+sharedstatedir ?= ${prefix}/com
+localstatedir ?= ${prefix}/var
+libdir ?= ${exec_prefix}/lib
+infodir ?= ${prefix}/info
 docdir = 
-includedir = ${prefix}/include
+includedir ?= ${prefix}/include
 oldincludedir = /usr/include
-mandir = ${prefix}/share/man
+mandir ?= ${prefix}/share/man
 
 pkgdatadir = $(datadir)/$(PACKAGE)
 pkglibdir = $(libdir)/$(PACKAGE)
diff --git a/bin/Makefile b/bin/Makefile
index 52c1f50..abef96a 100644
--- a/bin/Makefile
+++ b/bin/Makefile
@@ -1,22 +1,22 @@
 SHELL = /bin/sh
 top_srcdir = ..
 srcdir = ../bin
-prefix = /usr
-exec_prefix = ${prefix}
-
-bindir = ${exec_prefix}/bin
-sbindir = ${exec_prefix}/sbin
-libexecdir = ${exec_prefix}/libexec
-datadir = ${prefix}/share
-sysconfdir = ${prefix}/etc
-sharedstatedir = ${prefix}/com
-localstatedir = ${prefix}/var
-libdir = ${exec_prefix}/lib
-infodir = ${prefix}/info
+prefix ?= /usr
+exec_prefix ?= ${prefix}
+
+bindir ?= ${exec_prefix}/bin
+sbindir ?= ${exec_prefix}/sbin
+libexecdir ?= ${exec_prefix}/libexec
+datadir ?= ${prefix}/share
+sysconfdir ?= ${prefix}/etc
+sharedstatedir ?= ${prefix}/com
+localstatedir ?= ${prefix}/var
+libdir ?= ${exec_prefix}/lib
+infodir ?= ${prefix}/info
 docdir = 
-includedir = ${prefix}/include
+includedir ?= ${prefix}/include
 oldincludedir = /usr/include
-mandir = ${prefix}/man
+mandir ?= ${prefix}/man
 
 pkgdatadir = $(datadir)/$(PACKAGE)
 pkglibdir = $(libdir)/$(PACKAGE)
diff --git a/docs/Makefile b/docs/Makefile
index 0a41179..e128c85 100644
--- a/docs/Makefile
+++ b/docs/Makefile
@@ -1,22 +1,22 @@
 SHELL = /bin/sh
 top_srcdir = ..
 srcdir = ../docs
-prefix = /usr
-exec_prefix = ${prefix}
-
-bindir = ${exec_prefix}/bin
-sbindir = ${exec_prefix}/sbin
-libexecdir = ${exec_prefix}/libexec
-datadir = ${prefix}/share
-sysconfdir = ${prefix}/etc
-sharedstatedir = ${prefix}/com
-localstatedir = ${prefix}/var
-libdir = ${exec_prefix}/lib
-infodir = ${prefix}/info
+prefix ?= /usr
+exec_prefix ?= ${prefix}
+
+bindir ?= ${exec_prefix}/bin
+sbindir ?= ${exec_prefix}/sbin
+libexecdir ?= ${exec_prefix}/libexec
+datadir ?= ${prefix}/share
+sysconfdir ?= ${prefix}/etc
+sharedstatedir ?= ${prefix}/com
+localstatedir ?= ${prefix}/var
+libdir ?= ${exec_prefix}/lib
+infodir ?= ${prefix}/info
 docdir = 
-includedir = ${prefix}/include
+includedir ?= ${prefix}/include
 oldincludedir = /usr/include
-mandir = ${datadir}/man
+mandir ?= ${datadir}/man
 
 pkgdatadir = $(datadir)/$(PACKAGE)
 pkglibdir = $(libdir)/$(PACKAGE)