Fix build failure on macOS with --enable-shared

./spec/ruby/optional/capi/ext/array_spec.c can match with spec/%/ if
using GNU Make under version 3.81. make command installed on macOS is
3.81, so ruby can't be built with default make on macOS with
  --enable-shared option since bda56a03a6

defs/gmake.mk

@@ -360,6 +360,10 @@ spec/bundler/%: PHONY
 spec/bundler: test-bundler-parallel
 	$(Q)$(NULLCMD)
 
+# workaround to avoid matching non ruby files with "spec/%/" under GNU make 3.81
+spec/%_spec.c spec/%_spec.bundle:
+	$(NOOP)
+
 spec/%/ spec/%_spec.rb: programs exts PHONY
 	+$(RUNRUBY) -r./$(arch)-fake $(srcdir)/spec/mspec/bin/mspec-run -B $(srcdir)/spec/default.mspec $(SPECOPTS) $(patsubst %,$(srcdir)/%,$@)