Make ./configure fail when C++ compiler is missing (#732803)

Currently ./configure passes successfully even if a C++ compiler is not
installed.  The first time building the code fails is when make tries
to compile the first C++ source file and the compiler executable is not
found.

Also because there is no C++ compiler the autoconf generated test for
Gtk::Window::set_default_icon_name() will always fail, leaving
HAVE_SET_DEFAULT_ICON_NAME unset.  Therefore as well as installing a
C++ compiler, ./configure must be run again to correctly test for
set_default_icon_name().

This is as a result of a little known consequence of having AC_PROG_CC
proceed AC_PROG_CXX in the autoconf file configure.ac.
    Subject: AC_PROG_CXX behaviour when no C++ compiler is found
    http://lists.gnu.org/archive/html/bug-autoconf/2010-05/msg00001.html

Reverse the order, placing AC_PROG_CXX before AC_PROG_CC, so that the
generated ./configure script fails with an error if there is no C++
compiler available.

Bug #732803 - ./configure is successful even when C++ compile is missing