When mosquitto is included as subdirectory, `CMAKE_SOURCE_DIR` does not refer to the mosquitto top level CMake file, but to the whole project top level CMake.
Use `PROJECT_SOURCE_DIR` instead to refer to the right CMake in both contextes.
Signed-off-by: Pierre Hallot <hallotpierre@gmail.com>
This now handles the case where we are building from the released tar, or
building from git if xsltproc is available, or building from git if xsltproc
is not available.