Sometimes it is useful to use third-party m4 macros provided by some
build dependency (such as libgcrypt that provides an
AM_PATH_LIBGCRYPT macro).
In that situation we are introducing a dependency in bootstrap time, and it is not desirable: the dependencies should be checked in configure time.
Any third-party m4 file should be copied in libgnupdf/m4/ and put under version control. In that way we avoid the dependency in bootstrap time.