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>

configure.ac

@@ -637,6 +637,7 @@ m4_define([PROCESS_ADAPTERS], [
 			AC_MSG_ERROR([$3 is required for the ADAPTER_DESC([adapter])])
 		])
 		ADAPTER_VAR([adapter])=no
+		AC_DEFINE([BUILD_]ADAPTER_SYM([adapter]), [0], [0 if you do not want the ]ADAPTER_DESC([adapter]).)
 	])
 	AM_CONDITIONAL(ADAPTER_SYM([adapter]), [test "x$ADAPTER_VAR([adapter])" != "xno"])
   ])
@@ -756,6 +757,7 @@ AS_IF([test "x${gcc_wextra}" = "xyes"], [
   GCC_WARNINGS="${GCC_WARNINGS} -Wcast-align"
   GCC_WARNINGS="${GCC_WARNINGS} -Wredundant-decls"
   GCC_WARNINGS="${GCC_WARNINGS} -Wpointer-arith"
+  GCC_WARNINGS="${GCC_WARNINGS} -Wundef"
 ])
 AS_IF([test "x${gcc_werror}" = "xyes"], [
   GCC_WARNINGS="${GCC_WARNINGS} -Werror"