What happens if you remove line 45 from progs/xdemos/glxgears_fbconfig:
#include <GL/glxext.h>

I'm wondering if you have a glx.h file elsewhere that's defining
GLX_ARB_get_proc_address but not typdef'ing *__GLXextFuncPtr.

That's what happens :

[...]
make[2]: Entering directory `/tmp/Mesa-6.5/progs/xdemos'
gcc -Wall -Wmissing-prototypes -s -O3 -march=i686 -fPIC  -D_POSIX_SOURCE
-D_POSIX_C_SOURCE=199309L -D_SVID_SOURCE -D_BSD_SOURCE -DPTHREADS -DUSE_XSHM
-DUSE_X86_ASM -DUSE_MMX_ASM -DUSE_3DNOW_ASM -DUSE_SSE_ASM -I/usr/X11R6/include
-std=c99 -ffast-math  -c -I. -I../../include -Wall -Wmissing-prototypes -s -O3
-march=i686 -fPIC  -D_POSIX_SOURCE -D_POSIX_C_SOURCE=199309L -D_SVID_SOURCE
-D_BSD_SOURCE -DPTHREADS -DUSE_XSHM -DUSE_X86_ASM -DUSE_MMX_ASM -DUSE_3DNOW_ASM
-DUSE_SSE_ASM -I/usr/X11R6/include -std=c99 -ffast-math  glxgears_fbconfig.c
glxgears_fbconfig.c:53: error: parse error before "choose_fbconfig"
glxgears_fbconfig.c:53: warning: type defaults to `int' in declaration of
`choose_fbconfig'
glxgears_fbconfig.c:53: warning: initialization makes integer from pointer
without a cast
glxgears_fbconfig.c:53: warning: data definition has no type or storage class
glxgears_fbconfig.c:54: error: parse error before "get_visual_from_fbconfig"
glxgears_fbconfig.c:54: warning: type defaults to `int' in declaration of
`get_visual_from_fbconfig'
glxgears_fbconfig.c:54: warning: initialization makes integer from pointer
without a cast
glxgears_fbconfig.c:54: warning: data definition has no type or storage class
glxgears_fbconfig.c:55: error: parse error before "create_new_context"
glxgears_fbconfig.c:55: warning: type defaults to `int' in declaration of
`create_new_context'
glxgears_fbconfig.c:55: warning: initialization makes integer from pointer
without a cast
glxgears_fbconfig.c:55: warning: data definition has no type or storage class
glxgears_fbconfig.c: In function `init_fbconfig_functions':
glxgears_fbconfig.c:362: error: `PFNGLXCHOOSEFBCONFIGSGIXPROC' undeclared (first
use in this function)
glxgears_fbconfig.c:362: error: (Each undeclared identifier is reported only once
glxgears_fbconfig.c:362: error: for each function it appears in.)
glxgears_fbconfig.c:362: error: parse error before "glXGetProcAddressARB"
glxgears_fbconfig.c:364: error: `PFNGLXGETVISUALFROMFBCONFIGSGIXPROC' undeclared
(first use in this function)
glxgears_fbconfig.c:364: error: parse error before "glXGetProcAddressARB"
glxgears_fbconfig.c:366: error: `PFNGLXCREATECONTEXTWITHCONFIGSGIXPROC'
undeclared (first use in this function)
glxgears_fbconfig.c:366: error: parse error before "glXGetProcAddressARB"
glxgears_fbconfig.c:370: error: parse error before "glXGetProcAddressARB"
glxgears_fbconfig.c:372: error: parse error before "glXGetProcAddressARB"
glxgears_fbconfig.c:374: error: parse error before "glXGetProcAddressARB"
glxgears_fbconfig.c:383: warning: comparison between pointer and integer
glxgears_fbconfig.c:388: warning: comparison between pointer and integer
glxgears_fbconfig.c:393: warning: comparison between pointer and integer
glxgears_fbconfig.c: In function `make_window':
glxgears_fbconfig.c:432: error: invalid type argument of `unary *'
glxgears_fbconfig.c:445: error: invalid type argument of `unary *'
glxgears_fbconfig.c:470: error: invalid type argument of `unary *'
make[2]: *** [glxgears_fbconfig.o] Error 1
make[2]: Leaving directory `/tmp/Mesa-6.5/progs/xdemos'
make[1]: *** [subdirs] Error 1
make[1]: Leaving directory `/tmp/Mesa-6.5/progs'
make: *** [default] Error 1

Have you modified any of the Mesa/config/* files?  Looking at the gcc command:

gcc -Wall -Wmissing-prototypes -s -O3 -march=i686 [...]

I don't see where the "-s" and -march=i686 options are coming from, or why some
of the options are present twice.

A lot of build issues have been cleared up in the Mesa-7* releases. Can you try a more recent one and see if the issue persists?

Mesa 6.5 is no longer supported.  If you're still experiencing build errors with more recent versions of Mesa, please submit a new bug report.

Thanks.