Changes

debian/rules: move and rename flags

Spinal84 wrote:

1. DEB_CFLAGS_APPEND should be replaced with DEB_CFLAGS_MAINT_APPEND. Please check dpkg-buildflags(1) manual page (ENVIRONMENT section).
2. -DDEBUG should go to DEB_CPPFLAGS_MAINT_APPEND (it's C preprocessor option). (https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=923626)