Add customizable PREFIX in Makefile

The user of this package won't necessarily want to install to /usr/local
(indeed, that directory doesn't even exist on my system). Additionally,
package managers would probably like to install the program
elsewhere[1], and without a way to customize the prefix would have to fall back
to either patching the Makefile, or doing the installation manually, both
of which might break down the line.

By introducing a PREFIX variable with a default value of /usr/local, the
installation prefix can be customized, solving these problems.

[1]: Example: NixOS/nixpkgs#51004

Makefile

@@ -1,3 +1,4 @@
+PREFIX ?= /usr/local
 ROOT ?= $(shell pwd)
 
 test:
@@ -8,6 +9,6 @@ lint:
 	docker run --rm -v ${ROOT}:/mnt koalaman/shellcheck test/documentation-test
 
 install:
-	install src/semver /usr/local/bin
+	install src/semver ${PREFIX}/bin
 
 .PHONY: test install lint