cmake/FindIconv.cmake: fix return type in test code

[arjendekorte]

In openSUSE Tumbleweed, this test always fails because it compiles with -Werror=return-type by default. Fixing this by adding a return value in the test script to keep the compiler happy.

[micahsnyder]

Wow @arjendekorte I was going to work on this but you beat me to it! This is awesome. I'll give test before/after now. I'm also going to add opensuse docker containers to our internal CI pipeline, since it's clearly popular enough that we ought to add automated testing.

[arjendekorte]

Fixing this was rather easy (even for a CMake-novice like me), once I figured our where to find the CMakeError.log file. In the openSUSE Build Service, the build environment is discarded once the build fails and only the build log remains. After running a local build, it was clear what was going on and the fix is quite trivial.

[   41s] -- Performing Test Iconv_IS_BUILT_IN
[   41s] -- Performing Test Iconv_IS_BUILT_IN - Success

Adding openSUSE docker containers is probably a good idea for the Leap (stable) versions, but likely not so much for Tumbleweed. The latter is a rolling release with (almost) daily changes, so that would require a lot of maintenance to keep it up-to-date.

[micahsnyder]

I wasn't able to reproduce the issue with a default opensuse/tumbleweed:latest container.
I did reproduce it by building with:

cmake .. -D CMAKE_C_FLAGS="-Werror=return-type"

and confirmed that your PR fixes the issue.

[micahsnyder]

Merged to main and cherry-picked to the rel/0.104 branch so it gets into the next RC & release

[arjendekorte]

You probably couldn't reproduce this, because you didn't use the openSUSE Build Service (which is used for building all packages). By default, the CFLAGS are -O2 -Wall -D_FORTIFY_SOURCE=2 -fstack-protector-strong -funwind-tables -fasynchronous-unwind-tables -fstack-clash-protection -Werror=return-type -flto=auto -g -DNDEBUG, so this is where the error is from.