llvm.org.eclass: fix build w/o sphinx installed

Thanks to Ionen for reporting.

Fixes: 3bce8799e4
Signed-off-by: Sam James <sam@gentoo.org>

eclass/llvm.org.eclass

@@ -265,16 +265,17 @@ llvm.org_set_globals() {
 	fi
 
 	if [[ ${LLVM_MANPAGES} ]]; then
-		IUSE+=" doc"
 		# use pregenerated tarball if available
 		local manpage_dist=$(llvm_manpage_get_dist)
 		if [[ -n ${manpage_dist} ]]; then
+			IUSE+=" doc"
 			SRC_URI+="
 				!doc? (
 					https://dev.gentoo.org/~mgorny/dist/llvm/${manpage_dist}
 				)
 			"
 		else
+			IUSE+=" +doc"
 			# NB: this is not always the correct dep but it does no harm
 			BDEPEND+=" dev-python/sphinx"
 		fi
@@ -449,6 +450,15 @@ llvm_manpage_get_dist() {
 	fi
 }
 
+# @FUNCTION: llvm_manpage_dist_available
+# @DESCRIPTION:
+# Return true (0) if this LLVM version features prebuilt manpage
+# tarball, false (1) otherwise.
+llvm_manpage_dist_available() {
+	local manpage_dist=$(llvm_manpage_get_dist)
+	[[ -n ${manpage_dist} ]]
+}
+
 # @FUNCTION: llvm_are_manpages_built
 # @DESCRIPTION:
 # Return true (0) if manpages are going to be built from source,