add custom environment variable in bash source code

[b1431736437]

Configuration Information: ./configure --prefix=/data/data/com.termux/files/usr
Machine: aarch64
OS: linux-gnu
Compiler: clang
Compilation CFLAGS: -g -O2 -Wno-parentheses -Wno-format-security
uname output: Linux localhost 3.18.71-perf #1 SMP PREEMPT Wed Jul 27 22:01:17 
CST 2022 aarch64 Android
Machine Type: aarch64-unknown-linux-gnu

Bash Version: 5.1
Patch Level: 16
Release Status: release

Description:
        excuse me, how can I join the custom environment variable into the 
source code of Bash, and see this variable when execute env