build: Insist on m4 being installed.

build: Insist on m4 being installed.

Unlike stated in the manual, m4 is not an optional package that is
only needed on developer machines to build the html version of the
manual. Rather, the m4 macro processor is an essential requirement
because the gengetopt input files are generated with m4 from their
templates in m4/gengetopt.

This patch moves the documentation item on m4 from the development
section of the manual to the list of required packages for building
the paraslash package. It also adds a check for the m4 executable
to configure.ac to let the generated configure script fail the build
early on systems where m4 is not installed.