Created attachment 498060 [details, diff]
qtdeclarative 5.7.1 needs a very small patch to be built on uclibc-ng. A header is missing in the #include statements to have the perror function definition, which is included in stdio.h. There is no conditional here because perror is good old C89!
To apply it, add it to the PATCHES variable during the src_prepare phase inside the ebuild.