This is what I have changed in Makefile (for both: libnewgpio and gpioirq)
then just compile with make (libnewgpio and gpioirq consequently)
@Boken-Lin Thanks - had already found this. I think some of my issues to date have been to do with my mistakes in building kernel code using the OenWRT cross compilation - I think I had some settings wrong. Am in the process of trying again with git://git.openwrt.org/15.05/openwrt.git - will let you know how things go.