Bug 535972 deal with relative include paths in GCC scanner info

We weren't dealting with relative include paths returned from
the call to gcc -E properly. Resolves them against the build
directory before checking if their a directory. Results in them
added.

Tested with ESP-IDF's feature/cmake branch.

Change-Id: I1e2583e03cce6380ee4382146b177978c9c81806
1 file changed