[xml] threads.c compile failed with AIX 5.0

[田丙权 <tbq19 163 com>]

 hello:
    libxml 2.9.0, compile failed at threads.c with AIX 5.0.
"threads.c", line 918.20: 1506-046 (S) Syntax error.
"threads.c", line 918.20: 1506-277 (S) Syntax error: possible missing ';' or ','?
make[2]: *** [threads.lo] Error 1
make[2]: Leaving directory `/home/psfeed/tbq/libxml2-2.9.0'
make[1]: *** [all-recursive] Error 1
make[1]: Leaving directory `/home/psfeed/tbq/libxml2-2.9.0'
make: *** [all] Error 2