commit 3f89818

Allow to overwrite makefile variables

This allows user to overwrite make's default value. This is useful if
you want to install it into another folder, like with:

```
PREFIX=$HOME/.local make install
```

 1@@ -1,13 +1,13 @@
 2 .POSIX:
 3 .SUFFIXES:
 4 
 5-GO = go
 6-RM = rm
 7-SCDOC = scdoc
 8-GOFLAGS =
 9-PREFIX = /usr/local
10-BINDIR = bin
11-MANDIR = share/man
12+GO ?= go
13+RM ?= rm
14+SCDOC ?= scdoc
15+GOFLAGS ?=
16+PREFIX ?= /usr/local
17+BINDIR ?= bin
18+MANDIR ?= share/man
19 
20 all: senpai doc/senpai.1 doc/senpai.5
21