Improve static library support

[scpeters]

🦟 Bug fix

Port of gazebosim/sdformat#394 from @joxoby to ign-cmake2. This publicly defines the *_STATIC_DEFINE symbol for a library target if BUILD_SHARED_LIBS is Off.

Summary

I believe a special symbol needs to be defined when building with static libraries on Windows. Logic for defining this symbol was added to libsdformat11 in gazebosim/sdformat#394, and I noticed it while preparing to forward-port gazebosim/sdformat#780 to libsdformat11, since I noticed that ign-cmake2 doesn't currently define this symbol. I've added some examples using static libraries, but I'm not sure they're rich enough to prove this fixes any problems on Windows. Perhaps @traversaro @joxoby @madebr have some suggestions?

Checklist

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[traversaro]

I am currently away from laptop keyboard, but if I recall correctly this should not be necessary as the static case is already handled by the generate_export_header macro. Manually defining the static macro is only necessary when using the same header for both shared and static variants of a library (see https://cmake.org/cmake/help/latest/module/GenerateExportHeader.html) or when using an hardcoded export header file (as we were doing in sdformat if I recall correctly).

[traversaro]

I guess also @Ace314159 could be interested.

[traversaro]

I am currently away from laptop keyboard, but if I recall correctly this should not be necessary as the static case is already handled by the generate_export_header macro.

Related code: https://github.com/Kitware/CMake/blob/master/Modules/GenerateExportHeader.cmake#L302 .

[scpeters]

I am currently away from laptop keyboard, but if I recall correctly this should not be necessary as the static case is already handled by the generate_export_header macro. Manually defining the static macro is only necessary when using the same header for both shared and static variants of a library (see https://cmake.org/cmake/help/latest/module/GenerateExportHeader.html) or when using an hardcoded export header file (as we were doing in sdformat if I recall correctly).

ah, I believe you are right. I had noticed the *_STATIC_DEFINE macros, but didn't realize that it would generate the macros differently for static libraries. I will revert 9666346 and just keep the new examples. I'll also revert gazebosim/sdformat@72c4853 from gazebosim/sdformat#808, since I think it won't be needed either

[scpeters]

examples only in #202