make songbook.pdf :
- before patch: 5mn23s
- after patch: 14s
The issue was about LaTeX \graphicspath macro that was used for
covers. Since covers are stored in each artist's directory,
\graohicspath was containing a large number of different paths which
resulted in quite some time to evaluate when using \includegraphics.
This small patch copies all covers in a single temporary directory
before building a songbook. Note that symlinks are quite tricky to use
with pdflatex, esp. with MacOs/Windows plateforms so a hard copy has
been preferred.