Makefile: Allow prefix to be set from outside

This way external build systems can override prefix and decide to not
install into $HOME.

Signed-off-by: Alexander Dahl <ada@thorsis.com>
Fixes: c11923a ("make: add install and uninstall targets")

Makefile

@@ -1,4 +1,4 @@
-prefix = $(HOME)
+prefix ?= $(HOME)
 
 INSTALL_DIR = $(prefix)/share/sharness
 DOC_DIR = $(prefix)/share/doc/sharness