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dim
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svn:date
V 27
2020-02-21T06:58:07.636495Z
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V 545
devel/valgrind(-devel): Fix compiler version detection hopefully forever

Every time a compiler major version is bumped, we run into this exact
same error:

===>  Configuring for valgrind-3.10.1.20160113_7,1
...
checking for a supported version of gcc... no (10.0.0)
configure: error: please use gcc >= 3.0 or clang >= 2.9

Attempt to fix this for all versions in the future.

Other fix included: don't use undefined behavior to implement offsetof,
just use the official builtin function instead.

Approved by:	zeising (maintainer)
MFH:		2020Q1

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