* ng/var-simplify:
[ng] cosmetics: avoid redundant use of '&' in subroutine calls
[ng] rename: define_pretty_variable -> define_cond_variable()
[ng] cleanup: prefer 'define_variable' over 'define_pretty_variable'
[ng] refactor: change signature of 'define_variable()'
[ng] vars: get rid of VAR_ASIS / VAR_PRETTY distinction
[ng] vars: keep track of conditionals in appended values and comments
[ng] vars: simplify logic for appending conditionally
[ng] VarDef: store comments and values as a perl array
[ng] refactor: support comments only for VarDef, not for ItemDef too
[ng] tests: Automake should let us append to undefined variables (someday)
[ng] tests: spy behaviour of '+=' with GNU make
[ng] vars: get rid of VAR_SILENT type
[ng] vars: get rid of VAR_SORTED type
[ng] cleanup: remove unused private variables in Automake::Variable