dahdi: Add error if CONFIG_BKL is not defined

Might help when someone wonders why they are now getting errors about
"lock_kernel" being undefined.

Signed-off-by: Shaun Ruffell <sruffell@digium.com>
Acked-by: Kinsey Moore <kmoore@digium.com>

Origin: http://svnview.digium.com/svn/dahdi?view=rev&rev=9592

git-svn-id: http://svn.asterisk.org/svn/dahdi/linux/branches/2.4@9693 a0bf4364-ded3-4de4-8d8a-66a801d63aff

drivers/dahdi/dahdi-base.c

@@ -89,6 +89,12 @@
 #error "You cannot define both EMPULSE and EMFLASH"
 #endif
 
+#if LINUX_VERSION_CODE >= KERNEL_VERSION(2, 6, 37)
+#ifndef CONFIG_BKL
+#error You must have CONFIG_BKL lock defined to build DAHDI
+#endif
+#endif
+
 /* Get helper arithmetic */
 #include "arith.h"
 #if defined(CONFIG_DAHDI_MMX) || defined(ECHO_CAN_FP)