meson: Fix flex version parsing on Darwin platforms

This regression was introduced by commit 59c31d8d. On MacOS the `flex --version`
output has this format: flex version-number Apple-crap
So When splitting the version string we need to use the second field, not the last one.

Fixes #356
8 jobs for darwin-flex in 16 minutes and 28 seconds (queued for 2 seconds)
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passed #99929
gst indent

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manifest

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build fedora x86_64

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  Test
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check fedora

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  Full Builds
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build android arm64 api28

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build cerbero cross win32

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build cerbero cross win64

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build cerbero fedora x86_64

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