Added preprocessor checks and documentation to the bootloaders giving information about missing SIGNATURE_x defines due to outdated avr-libc versions.

Bootloaders/CDC/BootloaderCDC.h

@@ -48,6 +48,11 @@
 
 		#include <LUFA/Drivers/USB/USB.h>                // USB Functionality
 
+	/* Preprocessor Checks: */
+		#if !defined(SIGNATURE_0) || !defined(SIGNATURE_1) || !defined(SIGNATURE_2)
+			#error Device signature byte constants are not defined due to outdated avr-libc version. See demo documentation.
+		#endif
+
 	/* Macros: */
 		/** CDC Class Specific request to get the line encoding on a CDC-ACM virtual serial port, including the
 		 *  baud rate, parity, stop bits and data bits.

Bootloaders/CDC/BootloaderCDC.txt

@@ -50,9 +50,16 @@
  *
  *  <table>
  *   <tr>
- *    <td>
- *     None
- *    </td>
+ *    <td><b>Define Name:</b></td>
+ *    <td><b>Location:</b></td>
+ *    <td><b>Description:</b></td>
+ *   </tr>
+ *   <tr>
+ *    <td>SIGNATURE_0, SIGNATURE_1, SIGNATURE_2</td>
+ *    <td>Makefile CDEFS</td>
+ *    <td>AVR part signature bytes. These are normally defined as part of the AVR device header files in recent avr-libc
+ *        distributions. If your avr-libc library is out of date and does not define these values, you can define them
+ *        manually in the makefile CDEFS.</td>
  *   </tr>
  *  </table>
  */