Update translation pt_BR
New version of _dt_style_cleanup_multi_instance.

This fixes 2 issues:

- first some SQLite distributions are not compiled to support ORDER BY LIMIT
  on an UPDATE statement.

- second, the previous version were not properly working when there was
  multiple time the same multi_instance value for a style.

This new version is moving away from an SQL only implementation but will
be in turn easier to understand and maintain. We could revisit this
implementation if SQLite start supporting ROW_NUMBER.
tonecurve: display usual guides whatever histogram mode is chosen

The tonecurve module displays the histogram as its widget background
and currently changes the way guide lines are drawn depending on
histogram widget mode.

However the tonecurve module doesn't work in any new fancy histogram
way, so no use to take care about new guide lines for these modes.
Some fixes to WebP related build scripts

We were able to compile darktable when WebP wasn't found just because of
a bug in the WebP detection code.
Merge pull request #378 from edgomez/bugfix

tonecurve: display usual guides whatever histogram mode is chosen
Some more cmake cleanup
Merge pull request #379 from guitorri/up_pt_BR

Update translation pt_BR
Merge branch 'cleanup-multi-instance'

* cleanup-multi-instance:
  New version of _dt_style_cleanup_multi_instance.