cmake: Use proper build configuration during compiler/linker flag tests

[hebasto]

This PR makes all compiler/linker flag tests use the following build configuration:

• CMAKE_BUILD_TYPE with single-config generator, which means the same configuration during tests and the build.
• the default configuration, which is "RelWithDebInfo", with multi-config generators.

Addresses the issue raised during today's CMake-WG call.

[hebasto]

cc @fanquake @TheCharlatan @theuni

[theuni]

Lol. See comment here: #282 (comment)

I just came to the exact same conclusion :)

[theuni]

utACK other than the doc request.

[theuni]

Could you add a link to this here? This is arguably a bug that will likely be fixed in the future.

[hebasto]

Thanks! Done.

[m3dwards]

ACK 60d061a

Before PR with single config generators different flags were passed to TRY_COMPILE steps and after PR they matched. With multi config generators (I tested with Ninja) and before PR applied, no flags associated with build type were used with TRY_COMPILE steps. After PR - flags associated with RelWithDebInfo were seen.

It is worth noting that there is an (unavoidable?) subtle difference now between single config generator and multi config generators in that single config generators will align TRY_COMPILE flags with the actual compile flags. Whereas multi config generators will use different flags at compile time depending on build type selected and always flags associated with RelWithDebInfo for configuration time TRY_COMPILE checks.

[hebasto]

Rebased due to the recent sync/rebase cycle.

[m3dwards]

reACK e51c781

[theuni]

It is worth noting that there is an (unavoidable?) subtle difference now between single config generator and multi config generators in that single config generators will align TRY_COMPILE flags with the actual compile flags. Whereas multi config generators will use different flags at compile time depending on build type selected and always flags associated with RelWithDebInfo for configuration time TRY_COMPILE checks.

Agree this is annoying, but it's unfortunately just how CMake works. The lead dev says as much in the issue linked above. Ideally multi-config would do checks for each type (and even more ideally, in parallel), but this is just how it is for now I'm afraid.

[theuni]

utACK. I haven't tested, but this is what I expected.