This updates and fixes 487e5226 (setup: specify setuptools location
explicitly when calling make.) to use @kazuhiko original idea to
propagate only setuptools location. The reason is - when propagating
whole sys.path things break under tox tests:

---- 8< ----
========================================================================== test session starts ===========================================================================
platform linux2 -- Python 2.7.12, pytest-2.9.2, py-1.4.31, pluggy-0.3.1
rootdir: /home/kirr/src/wendelin/release/wendelin.core, inifile:
collected 33 items

bigarray/tests/test_arrayzodb.py .......
bigarray/tests/test_basic.py ........
bigfile/tests/test_basic.py ....
bigfile/tests/test_filefile.py .
bigfile/tests/test_filezodb.py ........
bigfile/tests/test_thread.py ....
lib/tests/test_calc.py .

======================================================================= 33 passed in 14.14 seconds =======================================================================
x86_64-linux-gnu-gcc -pthread -g -Wall -D_GNU_SOURCE -std=gnu99 -fplan9-extensions -Wno-declaration-after-statement -Wno-error=declaration-after-statement  -Iinclude -I3rdparty/ccan -I3rdparty/include   bigfile/tests/tfault.c lib/bug.c lib/utils.c 3rdparty/ccan/ccan/tap/tap.c  -o bigfile/tests/tfault.t
t/tfault-run bigfile/tests/tfault.t faultr on_pagefault
ok 1 - !pagefault_init()
Fatal Python error: Py_Initialize: Unable to get the locale encoding
  File ".../wendelin.core/.tox/py27-ZODB3-zblk0-fs-numpy110/lib/python2.7/encodings/__init__.py", line 123
    raise CodecRegistryError,\
                            ^
SyntaxError: invalid syntax

Current thread 0x00007f9b80024780 (most recent call first):
t/tfault-run: line 28: 21521 Аварийный останов         (core dumped) gdb -q -batch $tfault core > core.info
E: can't gdb(core)
Makefile:189: ошибка выполнения рецепта для цели «faultr.tfault»
make: *** [faultr.tfault] Ошибка 1
rm bigfile/tests/test_virtmem.t bigfile/tests/test_ram.t bigfile/tests/tfault.t bigfile/tests/test_pagemap.t
error: Failed to execute `make test`
ERROR: InvocationError: '.../wendelin.core/.tox/py27-ZODB3-zblk0-fs-numpy110/bin/python setup.py test'
________________________________________________________________________________ summary _________________________________________________________________________________
ERROR:   py27-ZODB3-zblk0-fs-numpy110: commands failed
---- 8< ----

What happens here is:

- gdb is used in automated tests
- gdb is linked with libpython3.5
- tox is currently running tests with python27
- setup.py sets PYTHONPATH to whole path from python27
- gdb, upon starting, initializes python runtime, which tries to load
  py27 modules under py35 -> oops.

So propagating only setuptools location practically solves the issue for
now, but still there is potential risk that in future, there will be
other modules put in the same location as setuptools (location is parent
dir of a module/package) which python tries to load on startup and which
might be incompatible between 2 & 3.

Thus not setting PYTHONPATH at all - e.g. creating environments
virtualenv way - would be better, but buildout fundamentally works the
other way. We can not also use -c 'buildout sys.path hack' in $PYTHON
itself, as $PYTHON is used in general way inside Makefile.

So let the hack with setuptools location in PYTHONPATH stay there until
it practically works.