scons: Make env['USE_PYTHON'] a source for createEnumStrings.
scons: Make env['USE_PYTHON'] a source for createEnumStrings.

That clues scons in to the fact that the contents of the target of that
function depends on the value of that construction variable.

Change-Id: I803322ea1a178152da3d95dffffe20acd4271c88
Reviewed-on: https://gem5-review.googlesource.com/3083
Reviewed-by: Jason Lowe-Power <68c46a606457643eab92053c1c05574abb26f861@lowepower.com>
Reviewed-by: Paul Rosenfeld <acc843c2779c7b68d219e057bff0dc0a66457419@micron.com>
Reviewed-by: Andreas Sandberg <c9f5686ee5bd561c93f9de79681eee75bfa7f36e@arm.com>
Maintainer: Andreas Sandberg <c9f5686ee5bd561c93f9de79681eee75bfa7f36e@arm.com>

gem5/gem5,powerjg/gem5-ci-test,Weil0ng/gem5,Weil0ng/gem5,sobercoder/gem5,rallylee/gem5,gedare/gem5,TUD-OS/gem5-dtu,powerjg/gem5-ci-test,TUD-OS/gem5-dtu,gem5/gem5,rallylee/gem5,gedare/gem5,sobercoder/gem5,sobercoder/gem5,rallylee/gem5,rallylee/gem5,gem5/gem5,sobercoder/gem5,rallylee/gem5,sobercoder/gem5,TUD-OS/gem5-dtu,powerjg/gem5-ci-test,Weil0ng/gem5,gedare/gem5,Weil0ng/gem5,powerjg/gem5-ci-test,TUD-OS/gem5-dtu,gem5/gem5,powerjg/gem5-ci-test,gedare/gem5,powerjg/gem5-ci-test,TUD-OS/gem5-dtu,rallylee/gem5,gedare/gem5,gem5/gem5,rallylee/gem5,gedare/gem5,gem5/gem5,Weil0ng/gem5,Weil0ng/gem5,TUD-OS/gem5-dtu,powerjg/gem5-ci-test,Weil0ng/gem5,gem5/gem5,sobercoder/gem5,sobercoder/gem5,gedare/gem5,TUD-OS/gem5-dtu