build: Clean up unnecessary complications

We don't need a custom Makefile for quiet rules: AM_V_GEN is pretty much
all we use anyway.

The Makefile.am.gtest file is not included any longer, since we switched
to TAP and GLib's own rules for unit testing.
build: Fix syntax error in --enable-man define

A missing comma was triggering a bad substitution and a warning when
running the configure script.
build: Drop unnecessary linker flags

The -no-undefined linker flag is needed only when compiling for Windows,
and it's added conditionally. Except that we also add it unconditionally
in the LDFLAGS used for the libjson-glib-1.0 shared object. Whoopsie.

We should also drop the unneeded -rpath: distributions will go out of
their way to patch it out in any case.
build: Move the pkg-config file under json-glib

The pkg-config file for a library is tied to the source, so it should
not be in the top-level of the project.
Clean up the README

Update the requirements, and clean up the text.
docs: Further clean ups of the README

Reword the description, and fit everything on a 80 columns margin.
docs: Add wiki link to the README file