meson: adjust whitespace handling in jinja2 rendering

meson: adjust whitespace handling in jinja2 rendering

In 6abe882bae1bb12827ef395c60f21ab8bb1bc61b the renderer was made to
unconditionally append a newline to output. This works, but is ugly. A nicer
solution is to tell jinja2 to not strip the newline in the first place, via
keep_trailing_newline=True. It seems that the result is unchanged because all
our source files have exactly one trailing newline.

Also, enable lstrip_blocks=True. This would cause whitespace on the line before
an {%if block to be automatically stripped. It seems reasonable to enable that
if trim_blocks=True.

Overall, no change is expected, though I didn't test combinations of
configurations, so there might be a change in some cases. But now the rules of
rendering are more logical, e.g. we should be able to indent nested conditional
statements without getting unexpected whitespace in the output.