add specs testing configure :set_define option and fix the same

build_tests/configure/check_c_header_no_fail_set_define.rb

@@ -0,0 +1,7 @@
+configure do
+  check_c_header "not___found.h", fail: false, set_define: "HAVE_NOT___FOUND_H"
+end
+
+Rscons::Environment.new(echo: :command) do |env|
+  env.Object("simple.o", "simple.c")
+end

build_tests/configure/check_c_header_success_set_define.rb

@@ -0,0 +1,7 @@
+configure do
+  check_c_header "string.h", set_define: "HAVE_STRING_H"
+end
+
+Rscons::Environment.new(echo: :command) do |env|
+  env.Object("simple.o", "simple.c")
+end