Warn on undefined preprocessor symbols

Preprocessor directives like "#if SYMBOL" silently replace undefined or
misspelt symbols with 0, which makes configuration bugs hard to spot.
Compiler flag "-Wundef" prevents such errors.

Change-Id: I91b7ba2db02ef0c3c452d334601c53aebda4660e
Signed-off-by: R. Diez <rdiezmail-openocd@yahoo.de>
Reviewed-on: http://openocd.zylin.com/6238
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>