xClean | Approximate String Joins with Abbreviations

The first thing I'll say is that this answer relies on your using GNU make. You don't say for sure that you are but since you're using pattern rules I will assume so. Note that what you want to do is not possible with standard POSIX make (unless you want to write out explicit rules for every target to be built).

You need two things:

1. Tell your compiler what you want to do
2. Tell make what you want to do

For #1, by default the compiler will generate the output which is the input file name, minus the extension (e.g. .c) plus a specific extension (e.g. .as). Since your input file is lib/GPIO/gpio.c the compiler generates gpio.as. Most compilers support a -o option which allows you to rename the output: you'll have to check your compiler's documentation if this doesn't work. So, you'd want to write your rule: