Allow a postfix to be defined for the version number

A separate build number (etc.) version number postfix can now be
added to the build without having to modify source code files by
defining VERSION_STR_POSTFIX. This can be done, e.g., by adding
following line to .config:

CFLAGS += -DVERSION_STR_POSTFIX=\"-foo\"

src/common/version.h

@@ -1,6 +1,10 @@
 #ifndef VERSION_H
 #define VERSION_H
 
-#define VERSION_STR "0.8.x"
+#ifndef VERSION_STR_POSTFIX
+#define VERSION_STR_POSTFIX ""
+#endif /* VERSION_STR_POSTFIX */
+
+#define VERSION_STR "0.8.x" VERSION_STR_POSTFIX
 
 #endif /* VERSION_H */