Fix Issue 23225 - OpenBSD: cpp invocation cannot find files

[ibara]

A bug in the shell script that lives in /usr/bin/cpp eats the -o
flag in the CPP invocation, which breaks ImportC. This workaround
is to use /usr/libexec/cpp instead, which is the actual CPP binary.

[dlang-bot]

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	23225	major	OpenBSD: cpp invocation cannot find files

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + dmd#14267"