This change:
- Adds a check for the doxygen program to configure.
- Moves the Doxyfile from src/Doxyfile to doc/Doxyfile.in.
- Generates a doc/Doxyfile from doc/Doxyfile.in inserting
an absolute path to the source directory, so that it
also works when builddir != srcdir.
- Adds `make api` rule for running doxygen; it depends on
api/doxygen file which is never created to force the rule
to always run.