Make USE_OPENSSL and USE_NSS mutually exclusive

build/build_config.h

 // Copyright (c) 2006-2008 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // This file adds defines about the platform we're currently building on.
 //  Operating System:
 //    OS_WIN / OS_MACOSX / OS_LINUX / OS_POSIX (MACOSX or LINUX)
 //  Compiler:
 //    COMPILER_MSVC / COMPILER_GCC
 //  Processor:
@@ skipping 29 matching lines @@
 #endif
 
 // A flag derived from the above flags, used to cover GTK code in
 // both TOOLKIT_GTK and TOOLKIT_VIEWS.
 #if defined(TOOLKIT_GTK) || (defined(TOOLKIT_VIEWS) && !defined(OS_WIN))
 #define TOOLKIT_USES_GTK 1
 #endif
 
 #if defined(OS_LINUX) || defined(OS_FREEBSD) || defined(OS_OPENBSD) || \
     defined(OS_SOLARIS)
-#define USE_NSS 1  // Use NSS for crypto.
+#if !defined(USE_OPENSSL)
+#define USE_NSS 1  // Default to use NSS for crypto, unless OpenSSL is chosen.
+#endif
 #define USE_X11 1  // Use X for graphics.
 #endif
 
+#if defined(USE_OPENSSL) && defined(USE_NSS)
+#error Cannot use both OpenSSL and NSS
+#endif
+
 // For access to standard POSIXish features, use OS_POSIX instead of a
 // more specific macro.
 #if defined(OS_MACOSX) || defined(OS_LINUX) || defined(OS_FREEBSD) || \
     defined(OS_OPENBSD) || defined(OS_SOLARIS)
 #define OS_POSIX 1
 // Use base::DataPack for name/value pairs.
 #define USE_BASE_DATA_PACK 1
 #endif
 
 // Use tcmalloc
@@ skipping 49 matching lines @@
 // On Posix, we'll detect short wchar_t, but projects aren't guaranteed to
 // compile in this mode (in particular, Chrome doesn't). This is intended for
 // other projects using base who manage their own dependencies and make sure
 // short wchar works for them.
 #define WCHAR_T_IS_UTF16
 #else
 #error Please add support for your compiler in build/build_config.h
 #endif
 
 #endif  // BUILD_BUILD_CONFIG_H_