Add targets for documentation, tweak compiler flags.

* Add targets `docs` and `clean-docs`. `docs` simply invokes Doxygen.
* Change the value of CXXFLAGS to `-Wall -ggdb3 -O0`. That is all
  warnings, debug symbols for GDB and absolutely no optimalization (so
  that it doesn't interfere with debugging). Basically, it is a debug
  build.
* Change the ECHO calls so that all the headers have the same width
  (cometics).