Pass config macros via command line

The distributed files do not include `extconf.h` generated by
`create_header`.

Author: nobu

ext/digest/blake3/extconf.rb

@@ -92,7 +92,12 @@ def blake3_have_isa?(name, flag, snippet)
 
 $preload = %w[digest]
 
-create_makefile("digest/blake3")
+create_makefile("digest/blake3") do |mk|
+  mk.grep(/^CPPFLAGS *=/) {|m|
+    m.sub!(/(?=-DRUBY_EXTCONF_H)/) {[$defs, ''].join(' ')}
+  }
+  mk
+end
 
 # Emit one explicit compile rule per SIMD backend so each gets its own
 # instruction-set flag.  mkmf's implicit .c.o rule compiles every object with