Because automake used to delete depdirs at once (.deps) and there was an issue
with portability, curl's XC_AMEND_DISTCLEAN greps the Makefiles in an attempt
to build a list of all depfiles and delete them individually instead.
Since commit 08849db866b44510f6b8fd49e313c91a43a3dfd3, automake switched from
deleting directories to individual files. curl's custom logic now finds a lot
more results with the grep (the filtering of these results isn't great), which
causes a massive bloating of the Makefile in the order of O(n^2).
Also remove now-unused XC_AMEND_DISTCLEAN macro group
References: https://github.com/curl/curl/issues/9843
References: https://debbugs.gnu.org/cgi/bugreport.cgi?bug=59288
Reported-by: Ilmari Lauhakangas
Fixes#9843Closes#10661
73e9e6d767