'Include flags + macro definitions
>>> h = HeaderList([\'/dir1/a.h\', \'/dir1/b.h\', \'/dir2/c.h\'])
>>> h.cpp_flags
\'-I/dir1 -I/dir2\'
>>> h.add_macro(\'-DBOOST_DYN_LINK\')
>>> h.cpp_flags
\'-I/dir1 -I/dir2 -DBOOST_DYN_LINK\'
Returns:
str: A joined list of include flags and macro definitions'

@property
def cpp_flags(self):

    cpp_flags = self.include_flags
    if self.macro_definitions:
        cpp_flags += ('    ' + self.macro_definitions)
    return cpp_flags
