package/linux-headers: error if headers version is not set

Validate that BR2_DEFAULT_KERNEL_VERSION is set when required.

Fixes:
http://autobuild.buildroot.net/results/2b9/2b91cb82a290348843a2422c01b10e49f45be9ca

Signed-off-by: James Hilliard <james.hilliard1@gmail.com>
[Arnout: add BR_BUILDING condition]
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>

package/linux-headers/linux-headers.mk

@@ -33,6 +33,12 @@ LINUX_HEADERS_REPO_URL = $(call qstrip,$(BR2_KERNEL_HEADERS_CUSTOM_REPO_URL))
 LINUX_HEADERS_CIP =
 endif # BR2_KERNEL_HEADERS_AS_KERNEL
 
+ifeq ($(BR2_KERNEL_HEADERS_VERSION)$(BR_BUILDING),yy)
+ifeq ($(LINUX_HEADERS_VERSION),)
+$(error No kernel headers version set, check your BR2_DEFAULT_KERNEL_VERSION setting)
+endif
+endif
+
 # Compute LINUX_HEADERS_SOURCE and LINUX_HEADERS_SITE from the configuration
 ifeq ($(LINUX_HEADERS_CUSTOM_TARBALL),y)
 ifeq ($(BR_BUILDING),y)