We now build a second binary package lumail-debug.

  The debug-package contains the same configuration file
 but the binary is compiled with debugging symbols enabled.
  When compiling lumail-debug we output to a different
 filename.

  This allows the "real" and "debug" versions to be installed
 side-by-side.
  Ignore binary files from debian/
  Handle debug-symbols for the Debian package.
  The -debug package depends upon the main one.

  As per the debian-developers reference.