Support for dynamic libraries

Sometimes you have to use --enable-shared, or build a python that is supposed to live in a given --prefix folder.

portable-python tries to support those cases as well, by using tools such as install_name_tool on macos or patchelf on linux.

Notes on linux

patchelf was used instead of chrpath for 2 reasons:

• chrpath can only replace an RPATH with one of equal or shorter length
• chrpath can only work on exes/libs that already have an RPATH