build: leverage cmake and bazel to determine when to use OpenSSL API (#15101)

Author: scotthart

google/cloud/BUILD.bazel

@@ -270,7 +270,9 @@ cc_library(
             "WIN32_LEAN_AND_MEAN",
             "_WIN32_WINNT=0x0A00",
         ],
-        "//conditions:default": [],
+        "//conditions:default": [
+            "GOOGLE_CLOUD_CPP_HAVE_OPENSSL",
+        ],
     }),
     visibility = ["//:__subpackages__"],
     deps = [

google/cloud/google_cloud_cpp_rest_internal.cmake

@@ -146,6 +146,11 @@ if (WIN32)
     target_link_libraries(google_cloud_cpp_rest_internal PUBLIC ws2_32 bcrypt
                                                                 crypt32)
 else ()
+    # We already require OpenSSL for non-Windows platforms.
+    target_compile_definitions(
+        google_cloud_cpp_rest_internal
+        PUBLIC # Enable OpenSSL specific functionality.
+               GOOGLE_CLOUD_CPP_HAVE_OPENSSL)
     target_link_libraries(google_cloud_cpp_rest_internal PUBLIC OpenSSL::SSL
                                                                 OpenSSL::Crypto)
 endif ()

google/cloud/internal/curl_handle_factory.cc

@@ -17,7 +17,7 @@
 #include "google/cloud/internal/curl_options.h"
 #include "google/cloud/internal/make_status.h"
 #include "google/cloud/log.h"
-#ifndef _WIN32
+#ifdef GOOGLE_CLOUD_CPP_HAVE_OPENSSL
 #include <openssl/err.h>
 #include <openssl/ssl.h>
 #endif
@@ -30,7 +30,7 @@ namespace rest_internal {
 GOOGLE_CLOUD_CPP_INLINE_NAMESPACE_BEGIN
 namespace {
 
-#ifndef _WIN32
+#ifdef GOOGLE_CLOUD_CPP_HAVE_OPENSSL
 struct BIOPtrCleanup {
   int operator()(BIO* b) const { return BIO_free(b); }
 };
@@ -49,7 +49,7 @@ using SSL_CTX = void;
 #endif
 
 Status SetCurlCAInMemory(CurlHandleFactory const& factory, SSL_CTX* ssl_ctx) {
-#if _WIN32
+#ifndef GOOGLE_CLOUD_CPP_HAVE_OPENSSL
   return internal::InternalError(
       "SSL callback function currently not supported in windows",
       GCP_ERROR_INFO());