Use header searcher to find readline and linenoise.

mrbgems/mruby-bin-mirb/mrbgem.rake

@@ -3,7 +3,12 @@ MRuby::Gem::Specification.new('mruby-bin-mirb') do |spec|
   spec.author  = 'mruby developers'
   spec.summary = 'mirb command'
 
-  spec.linker.libraries << 'readline' if spec.cc.defines.include? "ENABLE_READLINE"
+  if spec.build.cc.search_header_path 'readline/readline.h'
+    spec.cc.defines << "ENABLE_READLINE"
+    spec.linker.libraries << 'readline'
+  elsif spec.build.cc.search_header_path 'linenoise.h'
+    spec.cc.defines << "ENABLE_LINENOISE"
+  end
 
   spec.bins = %w(mirb)
 end

mrbgems/mruby-bin-mirb/tools/mirb/mirb.c

@@ -22,7 +22,7 @@
 #define MIRB_WRITE_HISTORY(path) write_history(path)
 #define MIRB_READ_HISTORY(path) read_history(path)
 #define MIRB_USING_HISTORY() using_history()
-#elif ENABLE_LINENOISE
+#elif defined(ENABLE_LINENOISE)
 #define ENABLE_READLINE
 #include <linenoise.h>
 #define MIRB_ADD_HISTORY(line) linenoiseHistoryAdd(line)