Documented instructions do not work when using the C++ SDK in a shared library

[svepe]

Describe the bug
Cannot build a shared library using the the C++ SDK by following the default instructions
since the static library is built without position independent code.

To Reproduce
Steps to reproduce the behavior:

1. Follow these C++ SDK instructions and add it to a shared library project
2. Attempt to compile
3. Results in an error since the static library was not compiled with the -fPIC flag on

Fix
I managed to get things working by compiling from source with the following flags:

cmake -B build -S . -DCMAKE_BUILD_TYPE=Release -DCMAKE_CXX_FLAGS="-fpic"

I think it's a good idea to either add the flag by default (not sure what the repercussions are) or mention it in the documentation. Happy to look into either and submit a PR.

[Wumpf]

Afaik enabling position dependent code always/when not needed can hurt performance (see e.g. here and here), we don't want to turn it on always

I wonder if we could do something like this:

if(BUILD_SHARED_LIBS)
    set_property(TARGET rerun_sdk PROPERTY POSITION_INDEPENDENT_CODE ON)
endif()

in rerun_cpp/CMakeLists.txt (using CMake's POSITION_INDEPENDENT_CODE option which is a bit more robust & clearer imho)

But BUILD_SHARED_LIBS is a default setting so I'm not sure if that's the right thing to check for. Might work in some cases but not others.
Maybe there's a way to check whether the user requested rerun_sdk specifically as a shared library and then set the flag?

As you say maybe it's enough pointing this out in the docs - CMake documents this here as well btw. as a separate thing https://cmake.org/cmake/help/latest/guide/tutorial/Selecting%20Static%20or%20Shared%20Libraries.html

[svepe]

Given the possible performance issues, then mentioning it in the documentation should be enough. There is also this option for the case where FetchContent is used.