Add support to include ThunderSVM from another cmake project

[sbreuss]

Hi,

I wanted to use ThunderSVM as part of a different cmake project. This currently fails, as current master relies on

• CMAKE_BINARY_DIR
• PROJECT_SOURCE_DIR
• CMAKE_BINARY_DIR

I changed the usage of these variables to use the CURRENT versions with relative paths from there. With this changes, everything works as expected.