Makefile: add special target .DELETE_ON_ERROR

The special .DELETE_ON_ERROR deletes the target file on recipe error.
Otherwise, an incomplete output file may be considered up to date
the next time around. .DELETE_ON_ERROR provides reasonable
protection at virtually no cost.

Signed-off-by: R. Diez <rdiezmail-openocd@yahoo.de>
Change-Id: I67dca47ae5ddf3786993c87b9991b3046a85f00b
Reviewed-on: http://openocd.zylin.com/6235
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>

Makefile.am

@@ -2,6 +2,8 @@
 # have all needed files, that a GNU package needs
 AUTOMAKE_OPTIONS = gnu 1.6
 
+.DELETE_ON_ERROR:
+
 # make sure we pass the correct jimtcl flags to distcheck
 DISTCHECK_CONFIGURE_FLAGS = --disable-install-jim