lint(ShellCheck): fix SC2163 warning

According to ShellCheck's documentation:

export takes a variable name, but ShellCheck has noticed that you give it
an expanded variable instead.  Remove $/${} for that, or use ${var?} to quiet.

Related: https://www.shellcheck.net/wiki/SC2163
Related: #470

configure

@@ -837,7 +837,7 @@ for arg in "$@"; do
       echo_error "Unrecognized option \`$arg'" tryhelp;;
    *=*)
       echo -e "Export \`$arg' in environment" ;
-      export "$arg" ;;
+      export "${arg?}" ;;
    *)
       # type of system to build the program for (not needed here but useful
       # for true autoconf configure script below