Merge branch '2849_without_prefix'

* 2849_without_prefix:
  Ticket #2849: bad EXTHELPERSDIR substitution if --prefix is not used.

configure.ac

@@ -462,6 +462,10 @@ AC_SUBST(LIBS)
 dnl Libraries used only when building the mc binary
 AC_SUBST(MCLIBS)
 
+dnl ${prefix} and ${exec_prefix} are undefined here if --prefix is not used in command line
+dnl Let define ${prefix} and ${exec_prefix}
+test "x$prefix" = "xNONE" && prefix=$ac_default_prefix
+test "x$exec_prefix" = "xNONE" && exec_prefix='${prefix}'
 
 if test x${libexecdir} = x'${exec_prefix}/libexec'; then
     EXTHELPERSDIR=${prefix}/libexec/${PACKAGE}/ext.d