demo: Link against Harfbuzz

demo: Link against Harfbuzz

The font features demo started calling the Harfbuzz API directly
starting from commit 9de3b24c205f1c647292a490f92f21a776b931a4. Harfbuzz
is an implicit dependency of Pango on some platforms, but it's not part
of the public dependencies; this means that we cannot expect to link to
Pango and automatically get Harfbuzz symbols to link against —
especially when things like --as-needed are in play.

This change triggered build failures on non-Unix platforms, fixed by
commit 2a9967731a3b77b102ada633c39b5a35efc80957, as well as build
failures in Continuous, with this error message:

/usr/lib/gcc/x86_64-gnomeostree-linux/4.9.3/../../../../x86_64-gnomeostree-linux/bin/ld:
font_features.o: undefined reference to symbol 'hb_tag_to_string'
//lib/libharfbuzz.so.0: error adding symbols: DSO missing from command
line
collect2: error: ld returned 1 exit status

In order to get the font features demo to build everywhere we should
take an explicit, though optional, check on Harfbuzz, and conditionally
build the font features demo with the right compiler and linker flags.