bootstrap: simplify export_variables

Nameref attribute (local -n) is a Bash 4.3 feature, thus not available
on OSX with Bash 3.2.

And because we know the argument looks like a valid assignment, we can
pass it whole to export.

bootstrap.sh

@@ -139,8 +139,7 @@ function export_variables() {
     do
         if [[ "$arg" =~ ^[[:alpha:]][_[:alnum:]]*= ]]
         then
-            local -n var="${arg%%=*}"
-            export var="${arg#*=}"
+            export "$arg"
         else
             printf >&2 "bootstrap.sh: invalid argument: %s\n" "$arg"
             ret=1