graphics/darktable: Simplify options-dependent parts

Use variables prefixed by option names. This removes the need to use .if
blocks.

Use OPTIONS_SUB=yes and %%OPTION%% markers in pkg-plist. This fixes
pkg-plist for at least the LUA option [1].

Remove the post-patch target. It's not needed anymore, as the changes
were committed upstream.

Replace the usage of bsd.port.pre.mk/bsd.port.post.mk by
bsd.port.options.mk/bsd.port.mk.

Fix the build when WITH_DEBUG=1 is set. darktable's CMakeLists.txt
throws a fatal error if CMAKE_BUILD_TYPE is set to Debug and suggests to
use RelWithDebInfo: we now follow this advice.

Sort USES and options-prefixed variables alphabetically.

PR: 190516 [1]
Phabricator: D178
Submitted by: Matthieu Volat <mazhe@alkumuna.eu> (previous patch to fix [1])
Reviewed by: bapt@
Approved by: bapt@