build: fixed clang's warning when building openssl

clang doesn't seem to support 'Wno-old-style-declaration', this
is a work-around.

Fixes: #25550
Refs: nodejs/node-v0.x-archive#4186

PR-URL: #25954
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

deps/openssl/openssl_common.gypi

@@ -57,14 +57,17 @@
       ],
     }, {
       # linux and others
-      'cflags': ['-Wno-missing-field-initializers',
-                 ## TODO: check gcc_version>=4.3
-                 '-Wno-old-style-declaration'],
+      'cflags': ['-Wno-missing-field-initializers',],
       'defines': [
         'OPENSSLDIR="/etc/ssl"',
         'ENGINESDIR="/dev/null"',
         'TERMIOS',
       ],
+      'conditions': [
+        [ 'llvm_version==0', {
+          'cflags': ['-Wno-old-style-declaration',],
+        }],
+      ],
     }],
   ]
 }