parse-datetime.c generated in the source (instead of build) directory

[Kamil Dudka]

parse-datetime.c generated out of parse-datetime.y ends up in the source 
directory, instead of the build directory as one would expect.  This was 
introduced by the following commit:

http://git.savannah.gnu.org/gitweb/?p=gnulib.git;a=commitdiff;h=6c680191

Neither the comment, nor the change log entry explains why it is desired.
As far as I know, the build should work even if the source directory is
on a read-only file system.

Moreover, this oddity seems to cause problems to the debug info extractor
used by rpmbuild in Fedora:

https://bugzilla.redhat.com/1555079

Kamil