site: add xorg_cv_malloc0_returns_null

util-macros 1.18 will look in the autoconf cache for this value so instead of
using overrides in xorg-lib-common, use the per-libc site cache.

Alert readers may notice that the site file claims that glibc returns NULL from
malloc(), when the previous change (e628c8aba0189de30de2833882b9999ff3b6547a)
claimed that it didn't.  The previous change was incorrect, whilst malloc(0)
returns a valid pointer, realloc(p,0) does in fact return NULL, so the Xlib
wrapper functions are needed.

(From OE-Core rev: ecbf3f81f6d2929921e57066c2dd327d41d8e286)

Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/site/common-glibc

@@ -80,3 +80,6 @@ ac_cv_va_copy=${ac_cv_va_copy=yes}
 ac_cv___va_copy=${ac_cv___va_copy=yes}
 ac_cv_func_va_copy=${ac_cv_func_va_copy=yes}
 ac_cv_func___va_copy=${ac_cv_func___va_copy=yes}
+
+# Xorg
+xorg_cv_malloc0_returns_null=${xorg_cv_malloc0_returns_null=yes}

meta/site/common-uclibc

@@ -47,3 +47,6 @@ ac_cv_func___va_copy=${ac_cv_func___va_copy=yes}
 ac_cv_func_posix_getpwuid_r=${ac_cv_func_posix_getpwuid_r=yes}
 ac_cv_func_posix_getgrgid_r=${ac_cv_func_posix_getgrgid_r=yes}
 ac_cv_func_posix_getpwnam_r=${ac_cv_func_posix_getpwnam_r=yes}
+
+# Xorg
+xorg_cv_malloc0_returns_null=${xorg_cv_malloc0_returns_null=yes}