Properly detect background thread support on Darwin.
When cross-compile, the host type / abi should be checked to determine
background thread compatibility.
diff --git a/configure.ac b/configure.ac
index 0661005..abcd91d 100644
--- a/configure.ac
+++ b/configure.ac
@@ -2394,7 +2394,8 @@
dnl ============================================================================
dnl Enable background threads if possible.
-if test "x${have_pthread}" = "x1" -a "x${je_cv_os_unfair_lock}" != "xyes" ; then
+if test "x${have_pthread}" = "x1" -a "x${je_cv_os_unfair_lock}" != "xyes" -a \
+ "x${abi}" != "xmacho" ; then
AC_DEFINE([JEMALLOC_BACKGROUND_THREAD], [ ], [ ])
fi
