include_directories: Fix handling of empty or space-only entries

Since commit 0d46e9a0 (Store includes from the same include_directories
call together., 2013-01-20) we accidentally use such entries.  Fix the
code to drop them instead.  Update the IncludeDirectories test to cover
this case.

Reported-by: Christophe Giboudeaux <cgiboudeaux@gmx.com>
Merge topic 'fix-include_directories-whitespace-handling'

b8cc6f4 include_directories: Fix handling of empty or space-only entries