[please let me use this issue to create my own merge request, backport request, and so on ...]

harfbuzz headers are installed into a subdir in /usr/include on Debian/Ubuntu systems, e.g. /usr/include/harfbuzz/hb.h

libfontmanager then fails to build. Caused by a typo:

--- a/make/modules/java.desktop/lib/Awt2dLibraries.gmk
+++ b/make/modules/java.desktop/lib/Awt2dLibraries.gmk
@@ -523,7 +523,7 @@ LIBFONTMANAGER_EXTRA_HEADER_DIRS := \
     libawt/java2d/loops \
     #
 
-LIBFONTMANAGER_CFLAGS += $(LIBFREETYPE_CFLAGS) $(HARFBUZZ_FLAGS)
+LIBFONTMANAGER_CFLAGS += $(LIBFREETYPE_CFLAGS) $(HARFBUZZ_CFLAGS)
 BUILD_LIBFONTMANAGER_FONTLIB += $(LIBHARFBUZZ_LIBS) $(LIBFREETYPE_LIBS)
 
 LIBFONTMANAGER_OPTIMIZATION := HIGH

Could be a candidate for 11u as well.