Do not use the extconf.h cache for OpenSSL when OPENSSL_PREFIX is set

* Ideally we would record the value of OPENSSL_PREFIX and compare, but
  extconf.h does not record that.
* See #2660

Author: eregon

lib/truffle/truffle/openssl-prefix.rb

@@ -8,6 +8,10 @@
 
 # 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

@@ -110,7 +110,7 @@ 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__ )
+if File.exist?(extconf_h) && File.mtime(extconf_h) >= File.mtime(__FILE__ ) && !Truffle::OPENSSL_PREFIX_WAS_SET
   $extconf_h = extconf_h
 else
 ### START of checks