build: fix ChangeLog generation in external build directory

build: fix ChangeLog generation in external build directory

When gitlog-to-changelog is run in a build directory that is neither the same
as, nor a subdirectory of, the source directory, it needs a --srcdir option or
it will fail to generate the changelog.  For instance

$ git clone https://git.savannah.gnu.org/git/automake.git
…
$ (cd automake && ./bootstrap)
$ mkdir b-automake
$ cd b-automake
$ ../automake/configure
…
$ make dist
…
  GEN      ChangeLog
gitlog-to-changelog:../s-automake/.git-log-fix: unused entry: 22729165f6bb902daeb8a4d8e7cb06982390f327
gitlog-to-changelog:../s-automake/.git-log-fix: unused entry: 3b369e6bbe0fb6d7359398935706c87dd9375cb6
make[2]: *** [../automake/maintainer/maint.mk:48: ChangeLog] Error 1

 * maintainer/maint.mk (gitlog_to_changelog_options): Add --srcdir option.