try to get a good checksum for CCACHE_PCH_EXTSUM also with gcc

Its PCHs are always different even if the source is the same (timestamps
somewhere?). But if the output of gcc -E for that precompiled header
source is the same, then technically the .gch should be without a change.
So this makes ccache get hits even if the .gch gets rebuilt, as long
as ccache is new enough to support CCACHE_PCH_EXTSUM (3.5+).

Change-Id: I447bb4840047f23deed55e25de1794047a0a9998
Reviewed-on: https://gerrit.libreoffice.org/72705
Tested-by: Jenkins
Reviewed-by: Luboš Luňák <l.lunak@collabora.com>

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