Directly handle $\ line endings

Directly handle $\ line endings

Previously we used the fact that this line ending expanded to "$ "
which would then expand to the empty string.  This has problems if
you enable warnings for undefined variables, so directly implement
this special (but documented) trick in the GNU Make parser.

As a side-effect this also removes all previous whitespace when
in GNU Make mode (not in POSIX mode) just as it would without "$".

* src/misc.c (collapse_continuations): Check for "$\" and remove it.
* tests/scripts/variables/flavors: Add regression tests including
with previous whitespace, and escaped/unescaped "$"