The Buildroot autobuilder failed to build libupnpp with the following uClibc-based toolchain: http://autobuild.buildroot.org/toolchains/tarballs/br-arm-cortex-m4-full-2017.11-rc1.tar.bz2
Probably the change in commit [3f1132] fails to detect the triplet "arm-buildroot-uclinux-uclibcgnueabi" as a linux system.