Replace bash arithmetic with POSIX shell constructs, so that
configure works with a POSIX /bin/sh such as dash.

--- a/configure.ac
+++ b/configure.ac
@@ -250,12 +250,12 @@
 # only one "with_gtkX" option allowed
 num_gtk_args=0
 if test "$with_gtk4" = "yes" ; then
-   (( num_gtk_args += 1 ))
+   num_gtk_args=$((num_gtk_args + 1))
 fi
 if test "$with_gtk3" = "yes" ; then
-   (( num_gtk_args += 1 ))
+   num_gtk_args=$((num_gtk_args + 1))
 fi
-if (( $num_gtk_args > 1 )); then
+if test "$num_gtk_args" -gt 1; then
    AC_MSG_ERROR('cannot set multiple "with-gtkX" option')
 fi
 
@@ -263,7 +263,7 @@
 if test "$no_x" = "yes" ; then
    with_gtk3="no"
    with_gtk4="no"
-   if (( $num_gtk_args > 0 )); then
+   if test "$num_gtk_args" -gt 0; then
       AC_MSG_WARN('"with-gtkX" option is ignored when "no_x" option is set')
    fi
 fi