Fix handling of whitespae in preprocessor macros for API generator

Fix handling of whitespae in preprocessor macros for API generator

The apibuild.py script did not handle whitespace in preprocessor
macros, so it failed to detect constants declared with '# define'
instead of '#define'. Since we now correctly indent our public
header files, we have silently lost all constants from
libvirt-api.xml. This also caused us to not detect formatting
errors in constant docs