Use enum instead of "static const" in class scope

Technically, static const needs an out-of-class definition.  Eg:

  CXXLD    libharfbuzz-subset.la
Undefined symbols for architecture x86_64:
  "OT::FeatureVariationRecord::min_size", referenced from:
      bool OT::GSUBGPOS::subset<OT::PosLookup>(hb_subset_context_t*) constin libharfbuzz_subset_la-hb-subset.o
      bool OT::GSUBGPOS::subset<OT::SubstLookup>(hb_subset_context_t*) constin libharfbuzz_subset_la-hb-subset.o
  "OT::Record<OT::LangSys>::min_size", referenced from:
      OT::Script::subset(hb_subset_context_t*) constin libharfbuzz_subset_la-hb-subset.o
  "OT::IntType<unsigned short, 2u>::min_size", referenced from:
      OT::Script::subset(hb_subset_context_t*) constin libharfbuzz_subset_la-hb-subset.o
      OT::RecordListOf<OT::Feature>::subset(hb_subset_context_t*) const  in libharfbuzz_subset_la-hb-subset.o
ld: symbol(s) not found for architecture x86_64
collect2: ld returned 1 exit status
make[4]: *** [libharfbuzz-subset.la] Error 1
make[3]: *** [all-recursive] Error 1
make[2]: *** [all] Error 2
make[1]: *** [all-recursive] Error 1
make: *** [all] Error 2
Exited with code 2

src/hb-open-type.hh

diff --git a/src/hb-open-type.hh b/src/hb-open-type.hh
index 29afa94..ef18163 100644
--- a/src/hb-open-type.hh
+++ b/src/hb-open-type.hh
@@ -150,7 +150,7 @@
 
 /* Script/language-system/feature index */
 struct Index : HBUINT16 {
-  static const unsigned int NOT_FOUND_INDEX = 0xFFFFu;
+  enum { NOT_FOUND_INDEX = 0xFFFFu };
 };
 DECLARE_NULL_NAMESPACE_BYTES (OT, Index);