Add cmark dependency

`cmark` is a more robust library for constructing Markdown than
`lowdown`. It is also more portable. Unfortunately it doesn't have a
terminal mode, so we cannot get rid of lowdown yet.

The `--enable-markdown` flag is renamed to `--enable-lowdown`
accordingly.

Makefile.config.in

@@ -4,6 +4,7 @@ BOOST_LDFLAGS = @BOOST_LDFLAGS@
 BUILD_SHARED_LIBS = @BUILD_SHARED_LIBS@
 CC = @CC@
 CFLAGS = @CFLAGS@
+CMARK_LIBS = @CMARK_LIBS@
 CXX = @CXX@
 CXXFLAGS = @CXXFLAGS@
 CXXLTO = @CXXLTO@

configure.ac

@@ -362,21 +362,24 @@ PKG_CHECK_MODULES([RAPIDCHECK], [rapidcheck rapidcheck_gtest])
 PKG_CHECK_MODULES([NLOHMANN_JSON], [nlohmann_json >= 3.9])
 
 
+# Look for cmark library.
+PKG_CHECK_MODULES([CMARK], [libcmark], [CXXFLAGS="$CMARK_CFLAGS $CXXFLAGS"])
+
 # Look for lowdown library.
-AC_ARG_ENABLE([markdown], AS_HELP_STRING([--enable-markdown], [Enable Markdown rendering in the Nix binary (requires lowdown) [default=auto]]),
-  enable_markdown=$enableval, enable_markdown=auto)
-AS_CASE(["$enable_markdown"],
+AC_ARG_ENABLE([lowdown], AS_HELP_STRING([--enable-lowdown], [Enable Markdown rendering in the Nix binary (requires lowdown) [default=auto]]),
+  enable_lowdown=$enableval, enable_lowdown=auto)
+AS_CASE(["$enable_lowdown"],
   [yes | auto], [
     PKG_CHECK_MODULES([LOWDOWN], [lowdown >= 0.9.0], [
       CXXFLAGS="$LOWDOWN_CFLAGS $CXXFLAGS"
       have_lowdown=1
       AC_DEFINE(HAVE_LOWDOWN, 1, [Whether lowdown is available and should be used for Markdown rendering.])
     ], [
-      AS_IF([test "x$enable_markdown" == "xyes"], [AC_MSG_ERROR([--enable-markdown was specified, but lowdown was not found.])])
+      AS_IF([test "x$enable_lowdown" == "xyes"], [AC_MSG_ERROR([--enable-lowdown was specified, but lowdown was not found.])])
     ])
   ],
   [no], [have_lowdown=],
-  [AC_MSG_ERROR([bad value "$enable_markdown" for --enable-markdown, must be one of: yes, no, auto])])
+  [AC_MSG_ERROR([bad value "$enable_lowdown" for --enable-lowdown, must be one of: yes, no, auto])])
 
 
 # Look for libgit2.

flake.nix

@@ -234,9 +234,9 @@
 
         # Toggles some settings for better coverage. Windows needs these
         # library combinations, and Debian build Nix with GNU readline too.
-        buildReadlineNoMarkdown = forAllSystems (system:
+        buildReadlineNoLowdown = forAllSystems (system:
           self.packages.${system}.nix.override {
-            enableMarkdown = false;
+            enableLowdown = false;
             readlineFlavor = "readline";
           }
         );

package.nix

@@ -25,6 +25,7 @@
 , libgit2
 , libseccomp
 , libsodium
+, cmark
 , lowdown
 , mdbook
 , mdbook-linkcheck
@@ -78,7 +79,7 @@
 , enableGC ? true
 
 # Whether to enable Markdown rendering in the Nix binary.
-, enableMarkdown ? !stdenv.hostPlatform.isWindows
+, enableLowdown ? !stdenv.hostPlatform.isWindows
 
 # Which interactive line editor library to use for Nix's repl.
 #
@@ -236,7 +237,8 @@ in {
     sqlite
     xz
     ({ inherit readline editline; }.${readlineFlavor})
-  ] ++ lib.optionals enableMarkdown [
+    cmark
+  ] ++ lib.optionals enableLowdown [
     lowdown
   ] ++ lib.optionals buildUnitTests [
     gtest
@@ -293,7 +295,7 @@ in {
     (lib.enableFeature enableInternalAPIDocs "internal-api-docs")
     (lib.enableFeature enableManual "doc-gen")
     (lib.enableFeature enableGC "gc")
-    (lib.enableFeature enableMarkdown "markdown")
+    (lib.enableFeature enableLowdown "lowdown")
     (lib.enableFeature installUnitTests "install-unit-tests")
     (lib.withFeatureAs true "readline-flavor" readlineFlavor)
   ] ++ lib.optionals (!forDevShell) [