'Macro definitions
>>> h = HeaderList([\'/dir1/a.h\', \'/dir1/b.h\', \'/dir2/c.h\'])
>>> h.add_macro(\'-DBOOST_LIB_NAME=boost_regex\')
>>> h.add_macro(\'-DBOOST_DYN_LINK\')
>>> h.macro_definitions
\'-DBOOST_LIB_NAME=boost_regex -DBOOST_DYN_LINK\'
Returns:
str: A joined list of macro definitions'

@property
def macro_definitions(self):

    return '    '.join(self._macro_definitions)
