[pdf-devel] m4 files

[jemarch]

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.