Create more magical Makefile

[jpmens]

Create a slightly more intelligent Makefile which works with Gnu make and BSD make. (I do not want to hear cmake or autotools or the like, thank you.)

OS.exec = uname -s | tr '[A-Z]' '[a-z]'
OS = $(shell $(OS.exec))$(OS.exec:sh)
show:
        @echo $(OS)

/via

[jpmens]

$ sh -c '. /etc/os-release; echo $ID'
debian   
centos
alpine       # on 3.8.1

[jpmens]

I've a bit of a feeling as though this is way overkill for this small utility, but this first attempt works on darwin, freebsd, openbsd, and debian.

The install target is particularly messy; I pass the target on through the layers of _makewith the$(TARGET)` variable.