'Include flags
>>> h = HeaderList([\'/dir1/a.h\', \'/dir1/b.h\', \'/dir2/c.h\'])
>>> h.include_flags
\'-I/dir1 -I/dir2\'
Returns:
str: A joined list of include flags'

@property
def include_flags(self):

    return '    '.join([('-I' + x) for x in self.directories])
