Add --enable-autobuild option

This enables the automatic download and build of the pgplot library only if the
user provides the --enable-autobuild option.  This is more security conscious
as it requires the user's explicit permission (by providing --enable-autobuild)
to download and install this third party library.
Add --disable-extra-libs option

Some PGPLOT installations have incompletely linked PGPLOT libraries so linking
against libcpgplot requires also linking against other libraries such as
libpgplot, libX11, libpng, etc.  These extra checks are mostly benign for
PGPLOT installations that don't need them, but they can cause problems.
For example, mkmf uses "gcc" when checking for the Fortran libraries, but the
resulting Makefile uses "cc" when linking.  If "cc" uses a different library
path than "gcc", it is possible for the check to succeed but the link to fail.
This might have been due to a weird combination of factors in my system
(e.g. custom Ruby build using gcc installed via MacPorts and MacOS's cc), but
if it happened to me it could happen to others.  Adding the --disable-extra-lib
option provides a solution for this case.
Close files in cogen.rb

This is important for running cogen.rb from within Rakefile.  Without
explicitly closing the output file, a truncated version of rb_pgplot.c
may be packaged in the gem file before the rb_pgplot.c file gets closed
when the rake process exits.
Be smart about extra library checks

Try to find libcpgplot before looking for extra libs like x11, gfortran,
png, etc.  If the initial check for libcpgplot fails, then look for the
extra libraries and re-check for libcpgplot.  This removes the need for
the --enable-extra-libs option.
extconf.rb:
* need to check extra-libs before auto-build.
* need to define $found_lib.
* define methods to make program structure clear.
* merge extconf-auto.rb.
rm extconf-auto.rb