Fix OpenSSL::Engine build on Debian

On Debian 9 (“stretch”) the `OPENSSL_NO_STATIC_ENGINE` macro is not
defined, which causes all the `#if HAVE_ENGINE_LOAD_…` directives to
fail with `error: 'HAVE_ENGINE_LOAD_…' is not defined, evaluates to 0
[-Werror,-Wundef]` while building TruffleRuby.

We can accomplish the same thing with `#ifdef`, which (of course) works
fine when the `HAVE_ENGINE_LOAD…` macros are also undefined.

Upstreamed from oracle/truffleruby#2255, which fixed
oracle/truffleruby#2254.

ext/openssl/ossl_engine.c

@@ -101,48 +101,48 @@ ossl_engine_s_load(int argc, VALUE *argv, VALUE klass)
         return Qtrue;
     }
     StringValueCStr(name);
-#if HAVE_ENGINE_LOAD_DYNAMIC
+#ifdef HAVE_ENGINE_LOAD_DYNAMIC
     OSSL_ENGINE_LOAD_IF_MATCH(dynamic, DYNAMIC);
 #endif
 #ifndef OPENSSL_NO_STATIC_ENGINE
-#if HAVE_ENGINE_LOAD_4758CCA
+#ifdef HAVE_ENGINE_LOAD_4758CCA
     OSSL_ENGINE_LOAD_IF_MATCH(4758cca, 4758CCA);
 #endif
-#if HAVE_ENGINE_LOAD_AEP
+#ifdef HAVE_ENGINE_LOAD_AEP
     OSSL_ENGINE_LOAD_IF_MATCH(aep, AEP);
 #endif
-#if HAVE_ENGINE_LOAD_ATALLA
+#ifdef HAVE_ENGINE_LOAD_ATALLA
     OSSL_ENGINE_LOAD_IF_MATCH(atalla, ATALLA);
 #endif
-#if HAVE_ENGINE_LOAD_CHIL
+#ifdef HAVE_ENGINE_LOAD_CHIL
     OSSL_ENGINE_LOAD_IF_MATCH(chil, CHIL);
 #endif
-#if HAVE_ENGINE_LOAD_CSWIFT
+#ifdef HAVE_ENGINE_LOAD_CSWIFT
     OSSL_ENGINE_LOAD_IF_MATCH(cswift, CSWIFT);
 #endif
-#if HAVE_ENGINE_LOAD_NURON
+#ifdef HAVE_ENGINE_LOAD_NURON
     OSSL_ENGINE_LOAD_IF_MATCH(nuron, NURON);
 #endif
-#if HAVE_ENGINE_LOAD_SUREWARE
+#ifdef HAVE_ENGINE_LOAD_SUREWARE
     OSSL_ENGINE_LOAD_IF_MATCH(sureware, SUREWARE);
 #endif
-#if HAVE_ENGINE_LOAD_UBSEC
+#ifdef HAVE_ENGINE_LOAD_UBSEC
     OSSL_ENGINE_LOAD_IF_MATCH(ubsec, UBSEC);
 #endif
-#if HAVE_ENGINE_LOAD_PADLOCK
+#ifdef HAVE_ENGINE_LOAD_PADLOCK
     OSSL_ENGINE_LOAD_IF_MATCH(padlock, PADLOCK);
 #endif
-#if HAVE_ENGINE_LOAD_CAPI
+#ifdef HAVE_ENGINE_LOAD_CAPI
     OSSL_ENGINE_LOAD_IF_MATCH(capi, CAPI);
 #endif
-#if HAVE_ENGINE_LOAD_GMP
+#ifdef HAVE_ENGINE_LOAD_GMP
     OSSL_ENGINE_LOAD_IF_MATCH(gmp, GMP);
 #endif
-#if HAVE_ENGINE_LOAD_GOST
+#ifdef HAVE_ENGINE_LOAD_GOST
     OSSL_ENGINE_LOAD_IF_MATCH(gost, GOST);
 #endif
 #endif
-#if HAVE_ENGINE_LOAD_CRYPTODEV
+#ifdef HAVE_ENGINE_LOAD_CRYPTODEV
     OSSL_ENGINE_LOAD_IF_MATCH(cryptodev, CRYPTODEV);
 #endif
     OSSL_ENGINE_LOAD_IF_MATCH(openssl, OPENSSL);