configure.sh: Tweak default of use_x86inc for 64-bit, non-pic, and Darwin.
configure.sh: Tweak default of use_x86inc for 64-bit, non-pic, and Darwin.

Mainly a readability fix.

Change-Id: Idfd4de9f25d9b9f9931d8e885d9d031473d5df6d

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