Make sure all ids in g_linkRefs are lower case

Ids created from the last line were not lower-cased, leading to the link
not being recognized.
Merge branch 'wip/fix-link-id-on-last-line' of https://github.com/agateau/doxygen into agateau-wip/fix-link-id-on-last-line

Conflicts:
	testing/055/md_055_markdown.xml
	testing/055_markdown.md