capnp: fix installation, build with fpic and build shared libraries

[timkpaine]

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capnproto is only building static libraries, and these are not actually linkable. This PR adds a small test case to ensure that the libraries can be linked, and also revamps to build both static and shared libraries (based off the bamtools.rb recipe).

xref: #139476
/usr/bin/ld: /home/linuxbrew/.linuxbrew/lib/libkj.a(exception.c++.o): relocation R_X86_64_TPOFF32 against _ZN2kj12_GLOBAL__N_1L19threadLocalCallbackE' can not be used when making a shared object; recompile with -fPIC

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[timkpaine]

The test added fails against the current build (you can see in commit c2aba45, /usr/bin/ld: /home/linuxbrew/.linuxbrew/Cellar/capnp/1.0.0/lib/libkj.a(exception.c++.o): relocation R_X86_64_TPOFF32 against _ZN2kj12_GLOBAL__N_1L19threadLocalCallbackE' can not be used when making a shared object; recompile with -fPIC , https://github.com/Homebrew/homebrew-core/actions/runs/5863531662/job/15897259150?pr=139577#step:4:103 )

I've squashed the commits now, but just wanted to show the error explicitly

[carlocab]

Thanks; could you squash your commits together and use the commit message

capnp: build with fpic and shared libraries

(or similar)?

[carlocab]

Thanks; but we shouldn't switch around the method of the installation of the shared and static libraries. Shared libraries should be installed using cmake --install.

Also, you'll need to do git fetch origin && git rebase origin/master to pull in the changes from #139592.

[carlocab]

Nice work; thanks for your contribution!

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