[Docs] Allow building man pages without myst_parser

The pan pages do not depend on the doc present in markdown files,
so they can be built without myst_parser.
Doing so might allow llvm distributions to have less development
dependencies.

As we do not have the ennvironment to test these configuration, this
capability is provided on a best-effort basis.

This restores a capability accidentally lost in llvm#65664.

clang/docs/conf.py

@@ -35,8 +35,16 @@
 
 import sphinx
 
-if sphinx.version_info >= (3, 0):
-    extensions.append("myst_parser")
+# When building man pages, we do not use the markdown pages,
+# So, we can continue without the myst_parser dependencies.
+# Doing so reduces dependencies of some packaged llvm distributions.
+try:
+  import myst_parser
+  extensions.append("myst_parser")
+except ImportError:
+  if not tags.has("builder-man"):
+    raise
+
 
 # The encoding of source files.
 # source_encoding = 'utf-8-sig'

flang/docs/conf.py

@@ -22,13 +22,23 @@
 # Add any Sphinx extension module names here, as strings. They can be extensions
 # coming with Sphinx (named 'sphinx.ext.*') or your custom ones.
 extensions = [
-    "myst_parser",
     "sphinx.ext.todo",
     "sphinx.ext.mathjax",
     "sphinx.ext.intersphinx",
     "sphinx.ext.autodoc",
 ]
 
+# When building man pages, we do not use the markdown pages,
+# So, we can continue without the myst_parser dependencies.
+# Doing so reduces dependencies of some packaged llvm distributions.
+try:
+  import myst_parser
+  extensions.append("myst_parser")
+except ImportError:
+  if not tags.has("builder-man"):
+    raise
+
+
 # Add any paths that contain templates here, relative to this directory.
 templates_path = ["_templates"]
 myst_heading_anchors = 6

llvm/docs/conf.py

@@ -26,7 +26,17 @@
 
 # Add any Sphinx extension module names here, as strings. They can be extensions
 # coming with Sphinx (named 'sphinx.ext.*') or your custom ones.
-extensions = ["myst_parser", "sphinx.ext.intersphinx", "sphinx.ext.todo"]
+extensions = ["sphinx.ext.intersphinx", "sphinx.ext.todo"]
+
+# When building man pages, we do not use the markdown pages,
+# So, we can continue without the myst_parser dependencies.
+# Doing so reduces dependencies of some packaged llvm distributions.
+try:
+  import myst_parser
+  extensions.append("myst_parser")
+except ImportError:
+  if not tags.has("builder-man"):
+    raise
 
 # Automatic anchors for markdown titles
 from llvm_slug import make_slug