Link libguicast with libpng

When trying to link a standalone with libguicast, I noticed that the
libguicast library refer to functions in the png library, but lack
-lpng on the link line.  This patch add -lpng to make sure the library
is linked with references to libpng, allowing programs to skip -lpng
when linking with libguicast.
Drop obsolete atexit() override in mwindow.C

In cinelerra/mwindow.C there is a reimplementation of the standard C
library function atexit().  The hack was most likely a workaround for
the problem reported in
<URL: https://www.sourceware.org/ml/libc-alpha/2001-05/msg00098.html >.

There is no code in Cinelerra using atexit() at the moment.  Removing
the hack to ensure the code build with the --with-lots-of-warnings
configure option, which detect conflicting atexit() signature in GNU
libc and Cinelerra.
Fix the last compiler warnings in guicast/.

Use 'const char *' instead of 'char *' where needed and fix a few formatting bugs.