my/m4/ax_check_docbook_xslt_min.m4: new
my/m4/ax_prog_xsltproc.m4: new
Merge branch 'master' of ../libmy
m4/: enable AX_PROG_XSLTPROC and AX_CHECK_DOCBOOK_XSLT_MIN macros
build system: switch from docbook2x-man to docbook 5 + xsltproc
doc/docbook/: make the nmsgtool.1 manpage build with docbook5 + xsltproc
README.md: add note about building the documentation
.travis.yml: add the docbook build-dependencies