Workaround bashisms in mk4build - need to evaluate for upstream

Without this patch, build fails with:
brltty-5.4/mk4build[20]: .[36]: local: not found [No such file or directory]
brltty-5.4/mk4build[20]: .[39]: local: not found [No such file or directory]
brltty-5.4/mk4build[20]: .[40]: local: not found [No such file or directory]
brltty-5.4/mk4build[20]: .[44]: local: not found [No such file or directory]
brltty-5.4/mk4build[20]: .[45]: readonly: =: invalid variable name


--- brltty-5.4/mk4build
+++ brltty-5.4/mk4build
@@ -1,4 +1,4 @@
-#!/bin/sh
+#!/bin/bash
 ###############################################################################
 # BRLTTY - A background process providing access to the console screen (when in
 #          text mode) for a blind person using a refreshable braille display.