ecm.eclass: Introduce _ecm_handbook_optional() to support BUILD_DOC

Signed-off-by: Andreas Sturmlechner <[email protected]>

eclass/ecm.eclass

@@ -595,6 +595,18 @@ ecm_src_prepare() {
 	fi
 }
 
+# @FUNCTION: _ecm_handbook_optional
+# @DESCRIPTION:
+# Use with ECM_HANDBOOK=optional; ticks either -DBUILD_DOC if available,
+# or -DCMAKE_DISABLE_FIND_PACKAGE_KF${_KFSLOT}DocTools
+_ecm_handbook_optional() {
+	if grep -Eq "option.*BUILD_DOC" CMakeLists.txt; then
+		echo "-DBUILD_DOC=$(usex handbook)"
+	else
+		echo "-DCMAKE_DISABLE_FIND_PACKAGE_KF${_KFSLOT}DocTools=$(usex !handbook)"
+	fi
+}
+
 # @FUNCTION: ecm_src_configure
 # @DESCRIPTION:
 # Wrapper for cmake_src_configure with extra logic for magic handling of
@@ -621,7 +633,7 @@ ecm_src_configure() {
 	fi
 
 	if [[ ${ECM_HANDBOOK} = optional ]] ; then
-		cmakeargs+=( -DCMAKE_DISABLE_FIND_PACKAGE_KF${_KFSLOT}DocTools=$(usex !handbook) )
+		cmakeargs+=( $(_ecm_handbook_optional) )
 	fi
 
 	if in_iuse designer && [[ ${ECM_DESIGNERPLUGIN} = true ]]; then