Any files with spaces in their filenames will break the recursive Build.mk search

[C0deH4cker]

This happened a few years ago in SunshineCTF, so it may be fixed already, but I don't think so. There was a file present with a name like Evil Spaces.pdf. Because of the space in the filename, the $(wildcard ...) call resulted in a broken "array", as arrays in make are space-separated. To avoid attempting to recurse into these broken path names, a simple existence check at the beginning of the recursive call should suffice. This will prevent recursing into directories whose names contain spaces, but at least a single file in the repo with a space in the name won't fully break PwnableHarness.