If a .c or .h file corresponds to a .y or .l file, skip indenting it.
There's no point in reindenting derived files, and these files tend to
confuse pgindent.  (Which probably indicates a bug in BSD indent, but
I can't get excited about trying to fix it.)

For the same reasons, add src/backend/utils/fmgrtab.c to the set of
files excluded by src/tools/pgindent/exclude_file_patterns.

The point of doing this is that it makes it safe to run pgindent over
the tree without doing "make maintainer-clean" first.  While these are
not the only derived .c/.h files in the tree, they are the only ones
pgindent fails on.  Removing that prerequisite step results in one less
way to mess up a pgindent run, and it's necessary if we ever hope to get
to the ease of running pgindent via "make indent".