Makefile.am: Move all code related to sources to `src/Makefile.am'.

* Makefile.am: Move all AM code related to sources to
  `src/Makefile.am'.
  (SUBDIRS): Add `src' directory.
* configure.ac (AC_CONFIG_FILES): Add `src/Makefile'.
* src/Makefile.am: New file.
* src/.gitignore: New file.