- reverted an upstream change that avoids writing symbols that are not
visible to .config, which breaks OpenWrt busybox's '.config' generation
logic.
- - add a compilation option (-DWARN_RECURSIVE_DEP) to treat recursive deps
- as a warning, avoiding a complete build failure because of unrelated or
- minor recursive deps, or making a scrict check before commiting a change
- that may cause one.
- use pre-built *.lex.c *.tab.[ch] files by default, to avoid depending on
flex & bison. Rebuild/remove these files only if running make with
BUILD_SHIPPED_FILES defined