Don't break help with gcc -O2

Don't break help with gcc -O2

libDtHelp is unable to read SDL help files
with -ftree-store-ccp optimization which
is enabled by -O2 on gcc 4.2.1.

GifUtils.c and decompress.c didn't work
properly with -ftree-store-ccp enabled.

GifUtils.c was repaired by fixing
those warnings:

GifUtils.c: In function 'create_pixmap':
GifUtils.c:1093: warning: return makes integer from pointer without a cast
GifUtils.c:1110: warning: return makes integer from pointer without a cast
GifUtils.c:1215: warning: return makes integer from pointer without a cast
GifUtils.c: In function 'gif_to_pixmap':
GifUtils.c:1242: warning: return makes integer from pointer without a cast

decompress.c didn't generate warnings, but the
only effect of the -ftree-store-cpp was to introduce
this change:

        addq    $1, %rax
        movq    %rax, (%rbx)
 .L90:
-       cmpl    $157, %edx
+       cmpl    $-99, %edx
        jne     .L86
        movl    8(%rbx), %eax
        subl    $1, %eax

Which corresponds to this source code:

bufioI.h

     57 #define BufFileGet(f)   ((f)->left-- ? *(f)->bufp++ : (*(f)->io) (f))

     42     int     (*io)(/* BufFilePtr f */);

decompress.c
     53 #ifdef NO_UCHAR
     54  typedef char   char_type;
     55 #else
     56  typedef        unsigned char   char_type;
     57 #endif /* UCHAR */
     58
     59 static  char_type magic_header[] = { "\037\235" };      /* 1F 9D */

    131     if ((BufFileGet(f) != (magic_header[0] & 0xFF)) ||
    132         (BufFileGet(f) != (magic_header[1] & 0xFF)))
    133     {
    134         return 0;
    135     }

BufFileGet() returns (int), so the (unsigned char) constants
got promoted to (int) with sign extension; therefore constant
157 decimal (0x9D) became -99 decimal, sign extended
(0xffffff9D), and the comparison was always false.

Tested using:
$ gcc -v
Using built-in specs.
Target: amd64-undermydesk-freebsd
Configured with: FreeBSD/amd64 system compiler
Thread model: posix
gcc version 4.2.1 20070831 patched [FreeBSD]

Running on:
FreeBSD 10.0-CURRENT (r240948M)
built Wed Sep 26 23:33:08 CEST 2012