exim: requires thread support

Fixes:

  http://autobuild.buildroot.net/results/98a1acb58665e6cd2df354fc63afef1d4606c6e0/

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/exim/Config.in

@@ -1,6 +1,10 @@
+comment "exim needs a toolchain w/ threads"
+	depends on !BR2_TOOLCHAIN_HAS_THREADS
+
 config BR2_PACKAGE_EXIM
 	bool "exim"
 	depends on BR2_USE_MMU # fork()
+	depends on BR2_TOOLCHAIN_HAS_THREADS
 	select BR2_PACKAGE_PCRE
 	select BR2_PACKAGE_BERKELEYDB
 	help