Offer libdir and mandir configure options. #504

I currently have a SlackBuild in pending for io.js, I am very excited to bring this great project to Slackware!

I would like to request this enhancement to packaging io.js, if feasible ... to get around being unable to change the library and man paths, I currently have this in the Slackware build script:

# Fix man page path.
sed -i 's|share/||' tools/install.py
sed -i '/var manRoot/ s/"share",//' deps/npm/lib/{,un}build.js

# Fix libdir for 64-bit.
sed -i "s|lib/|lib${LIBDIRSUFFIX}/|g" tools/install.py
sed -i "s/'lib'/'lib${LIBDIRSUFFIX}'/" lib/module.js
sed -i "s|\"lib\"|\"lib${LIBDIRSUFFIX}\"|" deps/npm/lib/npm.js

It'd be nice if io.js offered a typical configure flag option like --libdir and --mandir.

Thanks!