build: separate configuration and rules

_a)_ it's a good thing to separate these
_b)_ one can do this:
```
# cat libuv-config.mk
include config-common.mk

%:
	@echo $($@)
# make -I import/johndoe/libuv -f libuv-config.mk LDFLAGS
```

would it be of interest to add actual _`libuv-config.mk`_ from above?