build: Defer defining HAVE_PANGOFT and HAVE_HARFBUZZ

build: Defer defining HAVE_PANGOFT and HAVE_HARFBUZZ

We forgot to account for the case where we lookup for HarfBuzz manually
under Visual Studio builds, so only set HAVE_HARFBUZZ (and thus
HAVE_PANGOFT, since PangoFT2 depends on HarfBuzz) after we did the
fallback check for HarfBuzz.

Also, check for hb.h instead of harfbuzz/hb.h to be inline with the
pkg-config case, as the sources also include the HarfBuzz header by
using #include <hb.h>, not #include <harfbuzz/hb.h>

https://bugzilla.gnome.org/show_bug.cgi?id=773299