Use 'DESTDIR' for 'prefix'

* The Makefile uses the variable 'prefix' for what is normally 'DESTDIR'.
  Simplest workaround: set prefix from DESTDIR.
* Furthermore, the default value of 'prefix' was '/'. This causes a prefix
  of '//' in some cases which may be a problem in e.g. cygwin. No point
  keeping it.

--- a/Makefile
+++ b/Makefile
@@ -3,7 +3,7 @@
 # Builds "fxload" binaries, distribution tarball, and RPMs.
 #
 
-prefix =		/
+prefix =		$(DESTDIR)
 exec_prefix =		${prefix}
 sbindir =		${exec_prefix}/sbin
 mandir =		${prefix}/usr/share/man
