[ttfautohint-build.sh] eliminates backtick and expr syntax (shellcheck SC2006 and SC2003 warnings)

tools/scripts/install/ttfautohint-build.sh

@@ -1,7 +1,5 @@
 #!/bin/sh
 # shellcheck disable=SC2103
-# shellcheck disable=SC2003
-# shellcheck disable=SC2006
 # This script builds a stand-alone binary for the command line version of
 # ttfautohint, downloading any necessary libraries.
 #
@@ -84,21 +82,21 @@ count=0
 for i in $FREETYPE_PATCHES
 do
   curl -o ft-patch-$count.diff "$i"
-  count=`expr $count + 1`
+  count=$(($count + 1))
 done
 
 count=0
 for i in $HARFBUZZ_PATCHES
 do
   curl -o hb-patch-$count.diff "$i"
-  count=`expr $count + 1`
+  count=$(($count + 1))
 done
 
 count=0
 for i in $TTFAUTOHINT_PATCHES
 do
   curl -o ta-patch-$count.diff "$i"
-  count=`expr $count + 1`
+  count=$(($count + 1))
 done