qt-5.10: don't use features that require new-ish kernels, stay compat

Before this change:
$ readelf --notes /nix/store/zf5yja02g8n8dzgs25pqfd8w3myfzgzc-qtbase-5.10.1/lib/libQt5Core.so

Displaying notes found at file offset 0x004a7778 with length 0x00000020:
  Owner                 Data size       Description
  GNU                  0x00000010       NT_GNU_ABI_TAG (ABI version tag)
    OS: Linux, ABI: 3.17.0

After:
$ readelf --notes /nix/store/sg1s9hdw0b7p6h0dwg09g4lxy1acq7y6-qtbase-5.10.1/lib/libQt5Core.so

Displaying notes found at file offset 0x004a7dcc with length 0x00000020:
  Owner                 Data size       Description
  GNU                  0x00000010       NT_GNU_ABI_TAG (ABI version tag)
    OS: Linux, ABI: 2.6.28

-----------

The above paths were before rebasing the commit onto staging,
and it'd probably be good to have someone confirm the same happens
when built on a hydra builder or other non-dtzWill machine :).

[dezgeg: added comments]