Handling build errors

+++++++++++++++++++++

|ns3| releases are tested against the most recent C++ compilers available

in the mainstream Linux and MacOS distributions at the time of the release.

However, over time, newer distributions are released, with newer compilers,

and these newer compilers tend to be more pedantic about warnings.  |ns3|

configures its build to treat all warnings as errors, so it is sometimes

the case, if you are using an older release version on a newer system,

that a compiler warning will cause the build to fail.

For instance, ns-3.28 was released prior to Fedora 28, which included

a new major version of gcc (gcc-8).  Building ns-3.28 or older releases

on Fedora 28, when Gtk2+ is installed, will result in an error such as:

::

  /usr/include/gtk-2.0/gtk/gtkfilechooserbutton.h:59:8: error: unnecessary parentheses in declaration of ‘__gtk_reserved1’ [-Werror=parentheses]

   void (*__gtk_reserved1);

In releases starting with ns-3.28.1, an option is available in Waf to work

around these issues.  The option disables the inclusion of the '-Werror' 

flag to g++ and clang++.  The option is '--disable-werror' and must be

used at configure time; e.g.:

::

  ./waf configure --disable-werror --enable-examples --enable-tests

	opt.add_option('--disable-werror',

		       help=('disable -Werror flag (warnings treated as errors'),

		       default=False, dest='disable_werror', action="store_true")

        if Options.options.disable_werror:

                try:

                        warnings.remove ('-Werror')

                except ValueError:

                        pass