MFH: r526608

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: portmgr (joneum)
PR: 244234