extconf.rb: print informative message if OpenSSL can't be found

Describe --with-openssl-dir option that specifies the directory the
OpenSSL installed if the libraries could not found, and suggest
installing "development package" if just the headers are missing.

Also, let it raise an exception instead of writing to Logging and
terminating the process. This improves the look of the error message on
'gem install openssl' or 'make' (on the Ruby tree) failure.

Author: rhenium

ext/openssl/extconf.rb

@@ -43,13 +43,17 @@
     have_library("crypt32")
   end
 
-  result = have_header("openssl/ssl.h")
-  result &&= %w[crypto libeay32].any? {|lib| have_library(lib, "CRYPTO_malloc")}
+  result = %w[crypto libeay32].any? {|lib| have_library(lib, "CRYPTO_malloc")}
   result &&= %w[ssl ssleay32].any? {|lib| have_library(lib, "SSL_new")}
   unless result
-    Logging::message "=== Checking for required stuff failed. ===\n"
-    Logging::message "Makefile wasn't created. Fix the errors above.\n"
-    exit 1
+    raise "OpenSSL library could not be found. You might want to use " \
+      "--with-openssl-dir=<dir> option to specify the prefix where OpenSSL " \
+      "is installed."
+  end
+  unless have_header("openssl/ssl.h")
+    raise "OpenSSL library itself was found, but the necessary header files " \
+      "are missing. Installing \"development package\" of OpenSSL on your " \
+      "system might help."
   end
 end