2015-10-12: Don’t error, only warn if Python is missing.

Don’t error, only warn if Python is missing.

diff --git a/configure.ac b/configure.ac
--- a/configure.ac
+++ b/configure.ac
@@ -14,7 +14,7 @@ GUILE_PROGS([2.0.11])
 AC_ARG_VAR([python3], [How to call Python 3.])
 AC_CHECK_TARGET_TOOL([python3], [python3], [no])
 AS_IF([test "x$python3" = "xno"],
-      [AC_MSG_ERROR([cannot find Python 3 which is required for bootstrapping.])])
+      [AC_MSG_WARN([cannot find Python 3 which is required for development bootstrapping.])])
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