Make Bison version detection more portable

- POSIX sh actually does NOT support `+=`,
  but Bash does even in compatibility mode
- `mawk` does not fully support POSIX EREs (regexes),
  so work around its lack of brace support
Fixes building on Debian, apparently.

src/check_bison_ver.sh

@@ -1,7 +1,7 @@
 #!/bin/sh
-bison -V | awk '
-/^bison.*[0-9]+(\.[0-9]+){1,2}$/ {
-	match($0, /[0-9]+(\.[0-9]+){1,2}$/);
+bison -V | awk -v major="$1" -v minor="$2" '
+/^bison.*[0-9]+(\.[0-9]+)(\.[0-9]+)?$/ {
+	match($0, /[0-9]+(\.[0-9]+)(\.[0-9]+)?$/);
 	split(substr($0, RSTART), ver, ".");
 	if (ver[1] == major && ver[2] >= minor) { exit 0 } else { exit 1 }
-}' major="$1" minor="$2"
+}'