GTKTerm

The attached patch does the following:

- Removes the CFLAGS override to allow custom compiler
flags like any other program.

- Removes "-Wall -g" from CFLAGS; while I always use
"-W -Wall -Wformat=2" for GCC, hardcoding this is not
really nice. Not everybody uses GCC.

- Fixes all compiler warnings regarding unused parameters.

- Replaces all C++ style comments // with #if 0 ..
#endif sections. Several C compilers still do not
support C++ style comments. They're part of C99 but
they're not very useful in my opinion anyway.

- Make use of the environment variable "SHELL" if set
for spawning a new shell. Other terms do this as well
and it's in line with POSIX (See
http://www.opengroup.org/onlinepubs/009695399/basedefs/xbd_chap08.html).

- src/prefs.h uses DOS line termination, the patch
fixes that.

- Other minor mostly stylistic changes.