build: Do not generate ChangeLog from tarball.

* configure (main): Set .git-for-build.
* build-aux/install.sh.in (DESTDIR): Use it as canary to skip
ChangeLog rebuild.

build-aux/install.sh.in

@@ -56,7 +56,7 @@ mkdir -p ${DESTDIR}${docdir}
 if test -n "${PERL}"\
        && test -n "${GIT}"\
        && ${PERL} -v > /dev/null\
-       && ${GIT} status > /dev/null; then
+       && ! test -f .git-for-build; then
     ${PERL} ${srcdest}build-aux/gitlog-to-changelog --srcdir=. > ChangeLog+
 fi
 

configure

@@ -541,6 +541,7 @@ See \"Porting GNU Mes\" in the manual, or try --with-courage\n" mes-system)
           ;; Debian wants to run `make clean' from a tarball
           (and (zero? (system* "git" "init"))
                (zero? (system* "git" "add" "."))
+               (zero? (system* "touch" ".git-for-build"))
                (zero? (system* "git" "commit" "--allow-empty" "-m" "Import mes")))))
 
       (let ((pairs `(("@PACKAGE@" . ,PACKAGE)