Linux: add option to use system nspr.

base/third_party/nspr/nspr.gyp

+# Copyright (c) 2012 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.
+
+{
+  'conditions': [
+    ['use_system_nspr==1', {

[wtc, 2013/01/04 16:55:28]

In base/base.gyp, you added a dependency on this .gyp file
under the use_system_nspr==1 condition. So it seems
redundant to check the use_system_nspr==1 condition here.

[Paweł Hajdan Jr., 2013/01/04 18:01:31]

Actually, it's not! :)

Without this condition, gyp would execute command expansions below, which may
fail on systems which do not have nspr installed. Worse, I think it would
execute the commands on non-unix platforms, which don't have pkg-config.

Furthermore, command expansions would slow down gyp for everyone, for no reason
since use_system_nspr is not going to be used for Google Chrome.

In summary, this is a non-obvious consequence of how gyp works.

+      'targets': [
+        {
+          'target_name': 'nspr',
+          'type': 'none',
+          'toolsets': ['host', 'target'],
+          'variables': {
+            'headers_root_path': '.',
+            'header_filenames': [

[Ryan Sleevi, 2013/01/03 18:51:35]

And why not prcpucfg.h?

[wtc, 2013/01/04 16:55:28]

I don't understand how this works. Could you explain why we
need to list the headers but not the C/C++ source files?
Thanks.

[Paweł Hajdan Jr., 2013/01/04 18:01:31]

Thanks, added.

[Paweł Hajdan Jr., 2013/01/04 18:01:31]

Good question. See https://code.google.com/p/chromium/issues/detail?id=165264
for reference. Generally the idea is that when using system library we'd remove
all non-gyp files from the directory. To make the same #includes in .cc files
work, shim_headers.gypi uses a code generator to re-create the directory
structure in the build output directory and add forwarding headers there, which
just #include corresponding system headers. To let the generator know what paths
and file names to use (since the original files won't be there), we list them
here in variables.

Feel free to also take a look at shim_headers.gypi for reference. This solution
is intended to be transparent for people who work with the rest of the codebase.

[wtc, 2013/01/05 16:23:35]

Thank you for the explanation. This explains why this CL
doesn't need to update the
  #include "base/third_party/nspr/prtime.h"
line in base/time.cc.

+              'prtime.h',
+              'prtypes.h',
+            ],
+          },
+          'includes': [
+            '../../../build/shim_headers.gypi',
+          ],
+          'direct_dependent_settings': {
+            'cflags': [
+              '<!@(pkg-config --cflags nspr)',
+            ],
+          },
+          'link_settings': {
+            'ldflags': [
+              '<!@(pkg-config --libs-only-L --libs-only-other nspr)',
+            ],
+            'libraries': [
+              '<!@(pkg-config --libs-only-l nspr)',
+            ],
+          },
+        }
+      ],
+    }],
+  ],
+}