configure.ac already exists. Its timestamp doesn't need to be updated.
autoreconf takes care of the timestamps.
Since "foreign" is used in Makefile.am, no standard GNU files are
required. Distributing empty files like README could be disappointing for
the users.
Absolute symlinks break when the source is moved to another location. In
the best case, the symlink will be replaced with a copy, but that creates
code duplication. A user who got the packaged source may decide to change
one config.c without realizing that another config.c will get the same
changes.
Don't ignore config.c, it's a regular source file now, albeit very
short.
Testing done: compiled inside and outside build directory.