libcmd: add support for lowdown >= 3.0.0

lowdown 3.0.0 merged some flags into one to save on bits and did not add
any aliases for backward compatibility.

As with the changes for lowdown >= 1.4, we define a preprocessor flag to
gate the changes on and add a job to CI to ensure that lowdown < 3.0
keeps working (which is used by NixOS 25.11).

The channel version this Lix branch uses doesn't have bmake with support
for all tested platforms, so we can't properly build lowdown 3.0 (which
depends on it) for tests in CI. Given this is a relatively simple
backport tested on other Lix branches, it's probably fine.

Change-Id: I20a3e2fdaa05906f032ff66911c42867557fdd11
(cherry picked from commit e839708839aa132c8694abfdf83409a619f72c5a)

lix/libcmd/markdown.cc

@@ -74,7 +74,11 @@ std::string renderMarkdownToTerminal(std::string_view markdown, StandardOutputSt
         .vmargin = 0,
 #endif /* LOWDOWN_SEPARATE_TERM_OPTS */
         .feat = LOWDOWN_COMMONMARK | LOWDOWN_FENCED | LOWDOWN_DEFLIST | LOWDOWN_TABLES,
+#ifdef LOWDOWN_CONSOLIDATED_OFLAGS
+        .oflags = LOWDOWN_NOLINK,
+#else
         .oflags = LOWDOWN_TERM_NOLINK,
+#endif /* LOWDOWN_CONSOLIDATED_OFLAGS */
     };
     if (!shouldANSI(fileno)) {
         opts.oflags |= LOWDOWN_TERM_NOANSI;

meson.build

@@ -350,6 +350,11 @@ if lowdown.version().version_compare('>= 1.4.0')
   add_project_arguments('-DLOWDOWN_SEPARATE_TERM_OPTS', language: 'cpp')
 endif
 
+# TODO(sterni): drop the corresponding #ifdef after NixOS 25.11 is EOL which still distributes lowdown < 3.0.0
+if lowdown.version().version_compare('>= 3.0.0')
+  add_project_arguments('-DLOWDOWN_CONSOLIDATED_OFLAGS', language: 'cpp')
+endif
+
 # HACK(Qyriad): rapidcheck's pkg-config doesn't include the libs lol
 # Note: technically we 'check' for rapidcheck twice, for the internal-api-docs handling above,
 # but Meson will cache the result of the first one, and the required : arguments are different.