add AS_VAR_COPY for older autoconf versions

configure.ac

@@ -150,6 +150,16 @@ AM_COND_IF([ENABLE_PYTHON],
 	PKG_CHECK_MODULES([PYTHONDEV], [python3 >= 3.2],[],[AC_MSG_ERROR([You must install python3-dev])])
 	AC_DEFINE_UNQUOTED([ENABLE_PYTHON], 1, [Python3 is available])])
 
+# Not in older autoconf versions
+# AS_VAR_COPY(DEST, SOURCE)
+# -------------------------
+# Set the polymorphic shell variable DEST to the contents of the polymorphic
+# shell variable SOURCE.
+m4_ifdef([AS_VAR_COPY], [],
+[AC_DEFUN([AS_VAR_COPY],
+    [AS_LITERAL_IF([$1[]$2], [$1=$$2], [eval $1=\$$2])])
+])
+
 dnl PKG_CHECK_VAR was introduced with pkg-config 0.28
 m4_ifdef([PKG_CHECK_VAR], [],
 [AC_DEFUN([PKG_CHECK_VAR],