Fix icecc-create-env for relative paths

Sometimes the path where gcc finds liblto_plugin.so or specs file
is relative to its path. Therefore when gcc is added to the tarball
file in /usr/bin it looses the path to liblto_plugin.so or specs file.

Usually this bugs occurs to cross-compilling environments.

Signed-off-by: Ragner Magalhaes <ragner.magalhaes@gmail.com>