add --shrink-prefix used for build while package is not installed

[linxiulei]

FIX #100.

In addition, the '--shrink-prefix' option can be used for specify rpath when executable is building in anothen buildroot. For instance, if an executable located in /tmp/buildroot and can only be installed to /usr/local. To accomplish that in build stage, use:

$ patchelf --shrink-rpath --shrink-prefix /tmp/buildroot