Fix building on FreeBSD via cmake

[michael-yuji]

Currently building swift-testing via cmake (e.g. when building the toolchain) fail if not compile as PIC. Adding "-fPIC" fixed this problem. Moreover, we need to link against libexecinfo on FreeBSD where the symbol backtrace lives.

Result:

Swift testing can now successfully build via cmake thus installed to the toolchain 🥳

[grynspan]

I assume you've tested this?

[grynspan]

@swift-ci test

[michael-yuji]

I assume you've tested this?

yup, I compiled a toolchain with the change and tested on a random swift project (in this case, swift-crypto) and verified swift-testing is loading.

➜  swift-crypto git:(main) ✗ swift test --parallel
[1/1] Planning build
Building for debugging...
[1/1] Write swift-version-36984124FBC43BBA.txt
Build complete! (6.20s)
[293/293] Testing _CryptoExtrasTests.PBKDF2Tests/testRfcTestVectorsSHA1
◇ Test run started.
↳ Testing Library Version: 6.2-dev (faaabba3a977d67 - modified)
↳ Target Platform: x86_64-unknown-freebsd
✔ Test run with 0 tests passed after 0.001 seconds.

[grynspan]

We obviously don't have any CI set up for FreeBSD, but please wait until the CI we do have is done before merging.

[grynspan]

@swift-ci test

[grynspan]

Feel free to merge once CI is green—if you don't have the Big Green Button yet, send me a DM and I'll do it.

[grynspan]

@swift-ci test Windows