libedit fake-readline: provide empty tilde.h

otherwise gdb which includes "readline/tilde.h" will pull in its own
copy, which has prototypes (tilde_expand) that conflict with readline.h
from libedit. the issue seems to exist only in cross-compilation
scenarios.
libedit: make fit for crosscompilation
busybox: make fit for cross-compilation
gdb76: make fit for cross-compilation
butch template: make fit for crosscompilation

by allowing to override a couple vars.
a variable B which represents the build dir, formerly hardcoded as
/src/build, was introduced.
this goes hand in hand with the following butch commit
https://github.com/sabotage-linux/butch/commit/eb35445903b5b37ad3bc9d6373f4618c01e2a756

with the following config:
export PATH="$PATH":"$HOME"/musl-cross-4.8.3/mips-linux-musl/bin
export K=/src/KEEP
export S=/src
export B=$HOME/mips-prefix/src/build
export R=$HOME/mips-prefix
export C=/src/tarballs
export LOGPATH=$R/src/logs
export A=mips
export MAKE_THREADS=7
export CC="mips-linux-musl-gcc -L$R/lib -I$R/include"
export CROSS_COMPILE=mips-linux-musl-
export HOSTCC=gcc
export BUTCHDB=$R/var/lib/butch.db
export BUTCH_SKIPLIST=gcc3:stage0_gcc:stage0_musl:patch
export butch_filelists=$R/src/filelists/
export butch_tarballs_used=$R/src/tarballs_used.txt
export BUTCH_BUILD_TEMPLATE="$K/butch_template_configure_cached.txt"
export STAGE=1

i was able to crosscompile gdb and busybox like this:
CONFIG=config butch install busybox
(note: needs latest butch from git)
/src points to the local sabotage install; if you only have a git-checkout
you would therefore point S to the sabotage git dir, and K and C to the
respective subdirs.