[svn-r18530] Bug fix:

When gcc is used in the AIX platform, it failed to compile tconfig.c
because it does not recognize the __int64 type.  The ibm-aix has the
sizeof __int64=8 cached, as if all compilers supported this type.

Solution:
Removed the cached value for the sizeof __int64 so that during
configure, gcc can report it does not support it and not to use
it in the source code.

Tested:
NCSA Blue which is an AIX machine, using gcc or xlc compiler.
(The gcc still have failure but is being fixed as separated bugs.)

config/ibm-aix

@@ -91,7 +91,8 @@ ac_cv_sizeof_char=${ac_cv_sizeof_char=1}
 ac_cv_sizeof_short=${ac_cv_sizeof_short=2}
 ac_cv_sizeof_int=${ac_cv_sizeof_int=4}
 ac_cv_sizeof_long_long=${ac_cv_sizeof_long_long=8}
-ac_cv_sizeof___int64=${ac_cv_sizeof___int64=8}
+# Do not cache __int64 since it is not a standard C type and some compilers
+# (e.g., gcc) does not support it.
 ac_cv_sizeof_float=${ac_cv_sizeof_float=4}
 ac_cv_sizeof_double=${ac_cv_sizeof_double=8}
 ac_cv_sizeof_long_double=${ac_cv_sizeof_long_double=8}