gistfile1.sh

#!/bin/bash

      bash -c 'declare -p VAR'
VAR=1 bash -c 'declare -p VAR'
      bash -c 'declare -p VAR'

gistfile2.sh-session

bash: line 0: declare: VAR: not found
declare -x VAR="1"
bash: line 0: declare: VAR: not found

gistfile3.sh-session

$ CC=gcc CXX=g++ ./configure

gistfile4.sh-session

$ set | grep ^LANG=
LANG=en_US.UTF-8
$ LANG=C set | grep ^LANG=
LANG=en_US.UTF-8
$ (LANG=C; set) | grep ^LANG=
LANG=C

gistfile5.sh-session

$ LANG=C bash -c 'declare -p LANG'
declare -x LANG="C"
$ python -c 'import subprocess; subprocess.call(["LANG=C", "bash", "-c", "declare -p LANG"])'
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/usr/lib/python2.7/subprocess.py", line 522, in call
    return Popen(*popenargs, **kwargs).wait()
  File "/usr/lib/python2.7/subprocess.py", line 710, in __init__
    errread, errwrite)
  File "/usr/lib/python2.7/subprocess.py", line 1327, in _execute_child
    raise child_exception
OSError: [Errno 2] No such file or directory

gistfile6.sh-session

$ env LANG=C bash -c 'declare -p LANG'
declare -x LANG="C"
$ python -c 'import subprocess; subprocess.call(["env", "LANG=C", "bash", "-c", "declare -p LANG"])'
declare -x LANG="C"