build: show configure summary using a pretty format

[rbuj]

sample output:

Configure summary:

    mate-power-manager 1.26.0
    =========================

    prefix ......................: /usr
    datadir .....................: ${datarootdir}
    compiler ....................: gcc
    cflags ......................: 
    cwarnings ...................: -Wall -Wmissing-prototypes

    libsecret support ...........: yes
    gnome-keyring support .......: no
    Building extra applets ......: yes
    Self test support ...........: no
    dbus-1 services dir .........: ${datarootdir}/dbus-1/services

Now type `make' to compile mate-power-manager

[mbkma]

LGTM