Add version check for QNX systems before linking regex

[samkearney]

This is a follow up for #3713. For QNX RTOS, regex functionality was broken out into a separate library libregex as of version 7.1. However, in earlier versions, the same functionality is present in the core libc, and libregex doesn't exist. This causes problems when trying to configure GoogleTest using a toolchain for QNX < 7.1.0.

You can see the difference by comparing point 2 in the reference page for two different versions:

Version 7.0: http://www.qnx.com/developers/docs/7.0.0/#com.qnx.doc.ide.userguide/topic/writing_test_programs.html

Version 7.1: https://www.qnx.com/developers/docs/7.1/#com.qnx.doc.ide.userguide/topic/writing_test_programs.html

This PR allows GoogleTest to build successfully on QNX toolchains < 7.1.0.

cc @PhilipMourdjis the author of the previous PR that added this line.

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[samkearney]

@derekmauro anything I can do to help get this merged? Thanks!

[samkearney]

Just a friendly ping for this PR on its one-year anniversary. Thanks! 😄

[PhilipMourdjis]

It looks fine to me @derekmauro

[samkearney]

Closing, as a later identical PR #4444 was accepted.