CI: don't require Autoconf 2.71

.travis.yml

@@ -44,6 +44,8 @@ jobs:
         - git fetch --unshallow || true
         - git submodule update --init --recursive
 
+        # As of 2021-08, we don't have Autoconf 2.71 on Ubuntu.
+        - sed -i 's/AC_PREREQ(\[2\.71\])/AC_PREREQ([2.69])/g' configure.ac
         - ./bootstrap
         - ./configure --enable-gcc-warnings || { cat config.log && false; }
         - make -j2

TODO

@@ -517,6 +517,8 @@ https://github.com/scfc/bison-php/blob/master/data/lalr1.php
 https://lists.gnu.org/r/bison-patches/2013-09/msg00000.html and following
 
 ** Multiple start symbols
+Revert a70e75b8a41755ab96ab211a0ea111ac68a4aadd.
+
 Would be very useful when parsing closely related languages.  The idea is to
 declare several start symbols, for instance
 

configure.ac

@@ -15,11 +15,6 @@
 # You should have received a copy of the GNU General Public License
 # along with this program.  If not, see <https://www.gnu.org/licenses/>.
 
-# In order for some versions of Sun Studio to compile our C++ test cases
-# correctly, we need Autoconf 2.64 or better to handle the restrict
-# keyword in at least string.h from gnulib.  We need Autoconf 2.68 or
-# better to avoid a typo in the 'configure --help' entry for the YACC
-# environment variable.
 AC_PREREQ([2.71])
 m4_pattern_forbid([^_?(gl_[A-Z]|AX_|BISON_)])
 m4_pattern_allow([^BISON_USE_NLS$])