Handle incomplete versions in AX_SPLIT_VERSION

This ought to fix the error with clang that was triggered by
libMesh/TIMPI#53

Author: roystgnr

ax_split_version.m4

@@ -15,6 +15,7 @@
 #
 # LICENSE
 #
+#   Copyright (c) 2020 Roy Stogner
 #   Copyright (c) 2008 Tom Howard <[email protected]>
 #
 #   Copying and distribution of this file, with or without modification, are
@@ -27,8 +28,10 @@
 AC_DEFUN([AX_SPLIT_VERSION],[
     AC_REQUIRE([AC_PROG_SED])
     AX_MAJOR_VERSION=`echo "$VERSION" | $SED 's/\([[^.]][[^.]]*\).*/\1/'`
-    AX_MINOR_VERSION=`echo "$VERSION" | $SED 's/[[^.]][[^.]]*.\([[^.]][[^.]]*\).*/\1/'`
-    AX_POINT_VERSION=`echo "$VERSION" | $SED 's/[[^.]][[^.]]*.[[^.]][[^.]]*.\(.*\)/\1/'`
+    AX_MINOR_VERSION=`echo "$VERSION" | $SED -n 's/[[^.]][[^.]]*.\([[^.]][[^.]]*\).*/\1/p'`
+    AS_IF([test "x$AX_MINOR_VERSION" = "x"], [AX_MINOR_VERSION=0])
+    AX_POINT_VERSION=`echo "$VERSION" | $SED -n 's/[[^.]][[^.]]*.[[^.]][[^.]]*.\(.*\)/\1/p'`
+    AS_IF([test "x$AX_POINT_VERSION" = "x"], [AX_POINT_VERSION=0])
     AC_MSG_CHECKING([Major version])
     AC_MSG_RESULT([$AX_MAJOR_VERSION])
     AC_MSG_CHECKING([Minor version])