Allow usage as subproject

[madebr]

Hello,

While creating https://github.com/conan-io/conan-center-index/pull/1537, I noticed that easyhttpcpp cannot be used as a subproject.

This patch fixes this by replacing global cmake variables with project local ones.

• ${CMAKE_PROJECT_NAME} -> ${PROJECT_NAME}
• ${CMAKE_SOURCE_DIR} -> ${PROJECT_SOURCE_DIR}
• ${CMAKE_BINARY_DIR} -> ${PROJECT_BINARY_DIR}

I also introduced a cmake variable PROJECT_MODULE_PATH that contains the path to easyhttpcpp's cmake subdirectory.