see PR pending https://github.com/lanoxx/tilda/pull/545.patch
see also https://bugs.gentoo.org/927874
remove bashism
--- a/configure.ac
+++ b/configure.ac
@@ -186,7 +186,7 @@ cflagcount=0
 for flag in ${CFLAGS}; do
     cflaglines=$cflaglines$flag" "
     cflagcount=$(($cflagcount+1))
-    if (( $cflagcount % 4 == 0)); then
+    if test $((cflagcount % 4)) -eq 0; then
        dnl Here we concatenate the current cflaglines variable with a newline and enough spaces
        dnl such that the lines are correctly indented. Do not try to indent or reformat the
        dnl position of the double quotes (\").