I included the now needed libgcrypt.m4 file from the libgcrypt distribution into the m4/ subdirectory in the libgnupdf sources tree.
I think that it is desirable to have any needed m4 macro in the sources tree, allowing to build the autotools stuff without having all the dependencies installed. In this way we will get the "libgcrypt library not found" in configure time instead of the "underquote definition of AM_PATH_LIBGCRYPT" in bootstrap time. I am dropping a note about this in the hackers guide.
