Fix building issue on vcpkg

[Nukoooo]

I guess you forgot to add BUILD_TESTING check when making this change :^)

[jmigual]

Hi @Nukoooo I don't think I forgot. This is on purpose so the tests are build by default if this is the main project, i.e., if it's not an imported project. This code is extracted from the examples of the modern CMake book (https://gitlab.com/CLIUtils/modern-cmake/-/blob/master/examples/extended-project/CMakeLists.txt).

Why do you think it's an issue?

[Nukoooo]

Because I was trying to update the library in vcpkg with commit 0dc2f7a, and it threw this error:

CMake Error at test/CMakeLists.txt:13 (target_link_libraries):
  Target "sio_test" links to:

    Catch2::Catch2WithMain

From what I understand is that vcpkg pulls the source code and builds it as a main project, which passes the CMAKE_PROJECT_NAME STREQUAL PROJECT_NAME, hence the error is thrown. Currently I can't think of a better way to fix tihs problem but adding BUILD_TEST check

[Nukoooo]

Closing as someone created this PR microsoft/vcpkg#36226