xz: fix build without wchar support

xz itself works without wchar support, but the C99 test in configure
fails when it isn't available. Fix it by providing a cached value for
the test.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

package/xz/xz.mk

@@ -7,6 +7,7 @@ XZ_VERSION = 5.0.0
 XZ_SOURCE = xz-$(XZ_VERSION).tar.bz2
 XZ_SITE = http://tukaani.org/xz/
 XZ_INSTALL_STAGING = YES
+XZ_CONF_ENV = ac_cv_prog_cc_c99='-std=gnu99'
 
 $(eval $(call AUTOTARGETS,package,xz))
 $(eval $(call AUTOTARGETS,package,xz,host))