Reuse the openssl extconf.h only in development (when built by mx)

Author: eregon

lib/truffle/truffle/openssl-prefix.rb

@@ -8,10 +8,6 @@
 
 # Set OPENSSL_PREFIX in ENV to find the OpenSSL headers
 
-module Truffle
-  OPENSSL_PREFIX_WAS_SET = ENV.key?('OPENSSL_PREFIX')
-end
-
 search_homebrew = -> homebrew {
   if prefix = "#{homebrew}/opt/[email protected]" and Dir.exist?(prefix)
     prefix

src/main/c/openssl/extconf.rb

@@ -112,7 +112,8 @@ def find_openssl_library
 
 # TruffleRuby: do not perform all checks again if extconf.h already exists
 extconf_h = "#{__dir__}/extconf.h"
-if File.exist?(extconf_h) && File.mtime(extconf_h) >= File.mtime(__FILE__ ) && !Truffle::OPENSSL_PREFIX_WAS_SET
+in_development = ENV.key?('MX_HOME')
+if in_development && File.exist?(extconf_h) && File.mtime(extconf_h) >= File.mtime(__FILE__)
   $extconf_h = extconf_h
 else
 ### START of checks