Specify a julia binary instead of a julia prefix

and let julia collect the information about all other paths

configure.ac

@@ -458,13 +458,12 @@ AC_SUBST([READLINE_LIBS])
 dnl Find julia
 AC_ARG_WITH([julia],
   [AS_HELP_STRING([--with-julia@<:@=PREFIX@:>@],
-    [specify a prefix where julia can be found])],
+    [specify a julia binary])],
   [],[with_julia=no]
 )
 
 AS_IF([test "x$with_julia" != xno],[
-  AC_PATH_PROG([JULIA], [julia], [], [${with_julia}/bin ${PATH}])
-  AS_IF([test "x$JULIA" = x],[ AC_MSG_ERROR([no julia executable found]) ])
+  JULIA="${with_julia}"
 
   JL_SHARE=$($JULIA --startup-file=no -e 'print(joinpath(Sys.BINDIR, Base.DATAROOTDIR, "julia"))')
   AS_IF([test -f "${JL_SHARE}/julia-config.jl"], [], [AC_MSG_ERROR([no julia-config.jl found])])

etc/ci-prepare.sh

@@ -36,7 +36,7 @@ then
   cd julia-*
   JULIA_PATH=$(pwd)
   popd
-  CONFIGFLAGS="--with-gc=julia --with-julia=$JULIA_PATH $CONFIGFLAGS"
+  CONFIGFLAGS="--with-gc=julia --with-julia=${JULIA_PATH}/bin/julia $CONFIGFLAGS"
 fi