Added autoconf test

This is testing that the correct autoconf version has been used
to generate the ./configure file

.ci/install.autoconf

@@ -0,0 +1,19 @@
+#! /usr/bin/env bash
+
+set -e
+set -x
+
+cd "$(mktemp -dt plumed.XXXXXX)"
+
+dir=autoconf-$1
+
+wget https://ftp.gnu.org/gnu/autoconf/$dir.tar.gz
+
+tar xzf $dir.tar.gz
+
+cd $dir
+
+./configure --prefix "$HOME/opt"
+make
+make install
+

.github/workflows/ci.yml

@@ -189,6 +189,8 @@ jobs:
     - name: Install requirements
       run: |
         .ci/install.cppcheck 2.10
+        # it is important that this is the same version used to generate the actual ./configure
+        .ci/install.autoconf 2.69
     - name: Build astyle
       run: |
         make -j 4 -C astyle

configure.ac

@@ -5,11 +5,15 @@
 # system.
 # If you modify configure.ac, remember to run
 # autoconf and then commit both files to the git repository.
+# Make sure that you are using the same autoconf version that is
+# installed on GitHub Actions see `.github/workflows/ci.yml`
+# and search for `autoconf`. Otherwise, our codecheck job
+# on GitHub Actions will complain.
 
 # we require a recent version
 # notice that autoconf is not necessary on user's machine, but only
 # if one has to update configure.ac
-AC_PREREQ([2.68])
+AC_PREREQ([2.69])
 AC_INIT([PLUMED], [2])
 
 ##################################################################

src/maketools/plumedcheck

@@ -264,10 +264,9 @@ BEGINFILE{
 # DOC: When you modify the latter, the former should be regenerated using `autoconf` and
 # DOC: committed to git. This error indicates that the `./configure.ac` and `./configure` files
 # DOC: in the repository are not consistent.
-# DOC: Notice that Ubuntu 16 ships with an autoconf version that is not standard
-# DOC: (2.69 + some patch). This makes this test report a false positive.
-# DOC: This is thus changed to a style warning.
-    if(s!=0) style("autoconf","autoconf not satisfied")
+# DOC: Notice that this test should be performed using exactly the same autoconf version
+# DOC: that was used to generate the `./configure` file.
+    if(s!=0) error("autoconf","autoconf not satisfied")
     system("rm " tempfile)
   }