Fix misparsed $ after variable name

If a $ sign occurs after a variable name, the ninja string parser
fails to check if it is a $$ or a ${.  Go to the
parseDollarStartState instead of the parseDollarState.

Since it is not yet known if the $ is the beginning of a new
variable (${ or $<alphanumeric>) or a string ($$), an empty string
separator cannot be added to the ninjaString strings list.  Instead,
add functions to push strings or variables onto the ninjaString,
and automatically add the blank separator if two variables are
pushed in a row.

Change-Id: Ia1cae6259b1d7e4f633f61b9eadb2a2028bbd5f0