Remove use of stdbool.h in GDB sources.

Remove use of stdbool.h in GDB sources.

Using type bool from stdbool unfortunately causes problems trying
to build GDB on AiX and Solaris:

    In file included from ../../src/gdb/utils.h:24:0,
                     from ../../src/gdb/defs.h:707,
                     from ../../src/gdb/utils.c:20:
    /[...]/curses.h:96:14: error: two or more data types in declaration
    specifiers
     typedef char bool;
                  ^
    make[2]: *** [utils.o] Error 1

In theory, the problem is in curses.h which, in both cases, do
something similar. On Solaris:

    #if !defined(__cplusplus) && !defined(_BOOL)
    typedef char bool;
    #endif /* !defined(__cplusplus) && !defined(_BOOL) */

On AiX:

    #if !defined(__cplusplus) || (defined(__IBMCPP__) &&(__IBMCPP__<400))
    #ifndef _BOOL
    #define _BOOL
    typedef int bool;
    #endif
    #endif

You can reproduce the same problem by trying to compile:

    % cat toto.c
    #include <stdbool.h>
    #include <curses.h>
    % gcc -c toto.c
    In file included from toto.c:1:0:
    /[...]/curses.h:159:13: error: two or more data types in declaration
    specifiers
     typedef int bool;
             ^

This specific issue wouldn't occur if we included curses.h before
including stdbool.h, and I looked at that just to be complete.
Here is a small schematic representation of the include logic:

  * utils.c:
      -> defs.h -> utils.h -> stdbool.h
      -> gdb_curses.h -> curses.h

Because defs.h should always be first on the list, it means that
stdbool.h will always necessarily be included ahead of curses.h.

But, thinking beyond this very specific issue, it shows that using
stdbool.h is going to cause problems on these systems until either
GCC fixes those includes in a way that makes them work; or we switch
to C++.

In the meantime, I think the path of least resistance is to revert
the use of stdbool.h, and use integers, the way we've done up until
now. The benefits of using type "bool" are modest, IMO, so not
a great loss, and a temporary one.

gdb/ChangeLog:

        * utils.h: Remove <stdbool.h> #include.
        (producer_is_gcc): Change return type to "int".
        * utils.c (producer_is_gcc): Change return type to int.
        Return 1 instead of true, and 0 instead of false.
        Adjust function documentation accordingly.