Need to build from subdir of source without removing rest of repo

While attempting to build a library as a part, I need to follow these instructions:

    cd build/autotools
    autoreconf -i
    ./configure CFLAGS='-O2'
    make

The autotools plugin does almost exactly the right thing here, except that it doesn't cd into build/autotools first. I tried using the source-subdir option to make it build in that subdir, but unfortunately that's not meant to be the top of the source tree (for example, src/ is right next to build/ in this repo). Unfortunately source-subdir meant that all I had was my automake/m4 stuff without any C sources!

It would be extremely helpful if any of these build plugins had an option for a directory to cd into before either each step or the chain of steps as a whole.

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: snapcraft 2.13.1
ProcVersionSignature: Ubuntu 4.4.0-24.43-generic 4.4.10
Uname: Linux 4.4.0-24-generic x86_64
NonfreeKernelModules: zfs zunicode zcommon znvpair zavl
ApportVersion: 2.20.1-0ubuntu2.1
Architecture: amd64
CurrentDesktop: Unity
Date: Wed Jul 27 11:13:17 2016
InstallationDate: Installed on 2014-05-29 (789 days ago)
InstallationMedia: Ubuntu 14.04 LTS "Trusty Tahr" - Release amd64 (20140417)
PackageArchitecture: all
SourcePackage: snapcraft
UpgradeStatus: No upgrade log present (probably fresh install)