Gets rid of gcc 4.8 warnings:

dmxinputinit.c: In function ‘dmxInputScanForExtensions’:
dmxinputinit.c:877:14: warning: declaration of ‘display’ shadows a global declaration [-Wshadow]
     Display *display;
              ^
In file included from ../../../include/windowstr.h:60:0,
                 from dmxinputinit.c:72:
../../../include/opaque.h:52:30: warning: shadowed declaration is here [-Wshadow]
 extern _X_EXPORT const char *display;
                              ^

glxcmds.c: In function ‘__glXCreatePbuffer’:
glxcmds.c:3397:21: warning: declaration of ‘pc’ shadows a parameter [-Wshadow]
             CARD32 *pc = (CARD32 *) (be_req + 1);
                     ^
glxcmds.c:3314:52: warning: shadowed declaration is here [-Wshadow]
 __glXCreatePbuffer(__GLXclientState * cl, GLbyte * pc)
                                                    ^

glxscreens.c: In function ‘CalcServerVersionAndExtensions’:
glxscreens.c:139:35: warning: declaration of ‘req’ shadows a previous local [-Wshadow]
         xGLXQueryServerStringReq *req;
                                   ^
glxscreens.c:68:26: warning: shadowed declaration is here [-Wshadow]
     xGLXQueryVersionReq *req;
                          ^
glxscreens.c:140:36: warning: declaration of ‘reply’ shadows a previous local [-Wshadow]
         xGLXQueryServerStringReply reply;
                                    ^
glxscreens.c:69:27: warning: shadowed declaration is here [-Wshadow]
     xGLXQueryVersionReply reply;
                           ^

Signed-off-by: Alan Coopersmith <alan.coopersmith@oracle.com>
Reviewed-by: Rémi Cardona <remi@gentoo.org>
Reviewed-by: Keith Packard <keithp@keithp.com>