Avoid GNU seq(1)

MacOS, FreeBSD and Linux using GNU have it, but at least OpenBSD
does not (the non-compatible BSD equivalent jot(1) exists, though).

Counting up is easy enough to do in POSIX sh(1).

Looks like this after GNU make(1)'s escaping and produces the same
big-dynstr.c file:
```
cat main.c > big-dynstr.c
i=1; while [ $i -le 2000 ]; do echo "void f$i(void) { };"; i=$(($i + 1)); done >> big-dynstr.c
```

This is the last bit required to build and pass all tests on OpenBSD
without local patches/dependencies.

Author: klemensn

tests/Makefile.am

@@ -83,7 +83,7 @@ main_scoped_LDFLAGS = $(LDFLAGS_local)
 
 big-dynstr.c: main.c
 	cat $< > big-dynstr.c
-	for i in $$(seq 1 2000); do echo "void f$$i(void) { };"; done >> big-dynstr.c
+	i=1; while [ $$i -le 2000 ]; do echo "void f$$i(void) { };"; i=$$(($$i + 1)); done >> big-dynstr.c
 
 nodist_big_dynstr_SOURCES = big-dynstr.c
 big_dynstr_LDADD = -lfoo $(AM_LDADD)