undefined reference to `main' for shared library

I am trying to compile a C++ shared library with cmake. Here is a minimal reproducible example from this third-party code demonstrating the issue:

Content of CMakeLists.txt:

cmake_minimum_required(VERSION 3.5)
project(foobar)

set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} \
    -I/usr/include/c++/ \
    -I${CMAKE_INCLUDE_PATH} ")

add_library(foobar SHARED src/foobar.cpp)

Content of foobar.cpp:

class Foobar {
public:
    Foobar() {}
    virtual ~Foobar() {}
};

I am getting this linking error which doesn't make sense, since shared libraries don't need a main function:

$ cmake .
...
$ make
[ 50%] Building CXX object CMakeFiles/foobar.dir/src/foobar.cpp.o
[100%] Linking CXX shared library libfoobar.so
/usr/lib/gcc-cross/aarch64-linux-gnu/11/../../../../aarch64-linux-gnu/bin/ld: /usr/lib/gcc-cross/aarch64-linux-gnu/11/../../../../aarch64-linux-gnu/lib/../lib/Scrt1.o: in function `_start':
(.text+0x1c): undefined reference to `main'
/usr/lib/gcc-cross/aarch64-linux-gnu/11/../../../../aarch64-linux-gnu/bin/ld: (.text+0x20): undefined reference to `main'
collect2: error: ld returned 1 exit status
make[2]: *** [CMakeFiles/foobar.dir/build.make:97: libfoobar.so] Error 1
make[1]: *** [CMakeFiles/Makefile2:83: CMakeFiles/foobar.dir/all] Error 2
make: *** [Makefile:91: all] Error 2

With a slightly different flag order, I am getting this linking error:

relocation XYZ against `vtable for Foobar::~Foobar' can not be used 
when making a shared object; recompile with -fPIC

The issue is that the CMake variable CMAKE_INCLUDE_PATH is empty per default, and therefore adding -I${CMAKE_INCLUDE_PATH} to CMAKE_CXX_FLAGS causes g++ to be called with a flag -I missing a parameter for the directory path. g++ then interprets the next parameter -shared as the path of the include directory (linux directory names can start with a dash, so g++ has no way to know that the next parameter is not the name of the directory). Calling make VERBOSE=1 shows the issue:

aarch64-linux-gnu-g++ -fPIC -I/usr/include/c++/ -I -shared -Wl,-soname,libfoobar.so -o libfoobar.so CMakeFiles/foobar.dir/src/foobar.cpp.o

Thus g++ is not trying to compile a shared library, but an executable file, and complains about the lack of a main function.

A slightly different variant of this is when -fPIC is directly after -I while compiling the c++ file, then the linker complains that the c++ file was compiled without -fPIC and needs to be recompiled with -fPIC.