Only enforce build dir when not used as a subproject

[kinnarr]

Description
Allow in-source build when Catch2 is used as a subproject.

Additional context
Add NOT_SUBPROJECT to the if-statement

if (CMAKE_BINARY_DIR STREQUAL CMAKE_SOURCE_DIR)
    message(FATAL_ERROR "Building in-source is not supported! Create a build dir and remove ${CMAKE_SOURCE_DIR}/CMakeCache.txt")
endif()

[horenmar]

You should not be doing in-tree build anyway, but since the reason we check is that in-tree build leads to weird errors that should not be present when Catch2 is a subdirectory, I am willing to modify the check.