Fix compilation on travis with config.h.

`pkg-config --cflags thrift` on ubuntu adds the thrift include directory to
cflags. This directory contains a config.h that conflicts with ours. We just
rename ours to work around the issue.
Mark functions in paths.h as static.

These functions are static inline functions so we need to mark them as such to
avoid having multiple definitions at link time.
Add log_path() to paths.h.