xerces: fix buid with older cmake

Old version of cmake break cross-compilation when FindThreads is used
because of a try_run call. Add a workaround that avoids try_run.

cmake bug report: https://gitlab.kitware.com/cmake/cmake/issues/16920

Fixes:
 - http://autobuild.buildroot.org/results/784918c58310474e8b4a0b42850a55ba35a1a19e

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

package/xerces/xerces.mk

@@ -17,6 +17,11 @@ endef
 
 XERCES_POST_PATCH_HOOKS += XERCES_DISABLE_SAMPLES
 
+# Before CMake 3.10, passing THREADS_PTHREAD_ARG=OFF was needed to
+# disable a try_run() call in the FindThreads tests, which caused a
+# build failure when cross-compiling.
+XERCES_CONF_OPTS += -DTHREADS_PTHREAD_ARG=OFF
+
 ifeq ($(BR2_PACKAGE_ICU),y)
 XERCES_DEPENDENCIES += icu
 endif