CMAKE_MODULE_PATH is incorrectly set if paho.mqtt.cpp is compiled as a part of a bigger CMake project.

[vogoltsov]

If paho.mqtt.cpp is added to a bigger project (e.g. as a git submodule) and later included via add_subdirectory, the project fails to locate FindPahoMqttC.cmake.
This fix uses CMAKE_CURRENT_SOURCE_DIR instead of CMAKE_SOURCE_DIR to specify path to FindPahoMqttC.cmake relative to paho.mqtt.cpp'c root CMakeLists.txt.

[fpagliughi]

Thanks for the PR! It's probably better to work off the "develop" branch and submit commits back to "develop".

This issue was discussed in #172, and possibly fixed with 0d13c33 in the develop branch.

I'm not great with CMake, so let me know what you think of the different way I did it wit how you did it: PROJECT_SOURCE_DIR vs CMAKE_CURRENT_SOURCE_DIR.

[vogoltsov]

@fpagliughi Oh, sorry about the branch - I'm more used to gitlab flow. I will mind that for future PRs.

You are correct, PROJECT_SOURCE_DIR should work the same as the project() is defined in a top-level CMakeLists.txt. Thank you!