configure: add check for valid version

Problem: autoconf will accept junk at configure time as a valid
version, then go off and generate an invalid version.h file. This
happens frequently with shallow clones, setups in CI, or other
constrained user environments. It has caused new contributors a
lot of confusion in the past.

Solution: Like flux-framework/flux-sched#1219 suggested, reject
invalid versions at configure time and provide appropriate
suggestions to the user to remedy this.

configure.ac

@@ -35,6 +35,23 @@ AX_POINT_VERSION=$(echo $AX_POINT_VERSION | $SED 's/-.*$//')
 AC_SUBST([AX_MAJOR_VERSION])
 AC_SUBST([AX_MINOR_VERSION])
 AC_SUBST([AX_POINT_VERSION])
+if ! echo ${AX_MAJOR_VERSION} | grep -q [[0-9]][[0-9]]* ||
+   ! echo ${AX_MINOR_VERSION} | grep -q [[0-9]][[0-9]]* ; then
+    version_err_msg="
+    VERSION ${VERSION} is invalid.
+    Try the following to remedy this:
+
+      1. Set the variable manually, with FLUX_VERSION=<version>
+         in your environment.
+      2. If you are running in a CI environment, run \`git fetch tags\`
+         before building. Versions in flux-core are derived from
+         \`git describe\` which uses the most recent tag.
+      3. If you are running in a fork of the main repository, try
+         \`git push --tags\` to make sure tags are synchronized in
+         your fork.
+    "
+    AC_MSG_ERROR(["${version_err_msg}"])
+fi
 
 ##
 # Initialize pkg-config for PKG_CHECK_MODULES to avoid conditional issues