Makefile: add prefix_install target.
Makefile: add prefix_install target.

Add a Makefile target to do a more normal Unix-style prefix install.

abbeycode/Carthage,0xced/Carthage,abbeycode/Carthage,0xced/Carthage,flovilmart/Carthage,0xced/Carthage,erichoracek/Carthage,abbeycode/Carthage,erichoracek/Carthage,erichoracek/Carthage,erichoracek/Carthage,abbeycode/Carthage,NachoSoto/Carthage,NachoSoto/Carthage,NachoSoto/Carthage,flovilmart/Carthage,flovilmart/Carthage