configure: Add --with-pkg-extra-version=VER for packagers/distributions.

configure: Add --with-pkg-extra-version=VER for packagers/distributions.

This change adds a --with-pkg-extra-version option to ./configure to allow
packagers and distributions to fine tune the version displayed to the end
user to assist in support. It is also very useful when reporting bugs on
the official Quagga mailing lists.

There are two ways of utilsing this functionality:

      a) ./configure --with-pkg-extra-version=-wmo1
      b) EXTRAVERSION=-wmo1 ./configure

The latter is a common way for many distributions to add extra version
strings to signify their own patch level to a given package.

Also, minor whitespace fix in configure summary.

Signed-off-by: Joachim Nilsson <joachim.nilsson@westermo.se>
Signed-off-by: Joachim Nilsson <troglobit@gmail.com>
Signed-off-by: David Lamparter <equinox@opensourcerouting.org>