Without this line, doing a shared build will create

-- Installing: /home/stian/SORT/adplug/libbinio-cmake/_install/lib/cmake/libbinio/libbinio-static-targets.cmake
-- Installing: /home/stian/SORT/adplug/libbinio-cmake/_install/lib/cmake/libbinio/libbinio-static-targets-release.cmake

instead of

-- Installing: /home/stian/SORT/adplug/libbinio-cmake/_install/lib/cmake/libbinio/libbinio-shared-targets.cmake
-- Installing: /home/stian/SORT/adplug/libbinio-cmake/_install/lib/cmake/libbinio/libbinio-shared-targets-release.cmake

if invoked like this:
$ cmake -G Ninja -S . -B build-shared -DBUILD_SHARED_LIBS=YES -DCMAKE_BUILD_TYPE=Release
$ cmake --build build-shared
$ cmake --install build-shared --prefix _install

CMakeLists.txt

@@ -45,6 +45,7 @@ endif()
 if(DEFINED libbinio_BUILD_SHARED_LIBS)
   set(BUILD_SHARED_LIBS "${libbinio_BUILD_SHARED_LIBS}")
 endif ()
+set(libbinio_BUILD_SHARED_LIBS "${BUILD_SHARED_LIBS}")
 
 if(BUILD_SHARED_LIBS AND NOT DEFINED CMAKE_INTERPROCEDURAL_OPTIMIZATION)
   message(STATUS "Enabling interprocedural optimization")