What does this gyp syntax mean?

I have been digging up chromium code recently, then I found piece of code that I could not understand. File is located at src/media/media.gyp Can someone explain what this line means in plain English?

The line I don't understand :

 '<!@(<(pkg-config) --cflags libpulse)',

The code :

['OS!="linux"', {
      'sources!': [
        'audio/cras/audio_manager_cras.cc',
        'audio/cras/audio_manager_cras.h',
        'audio/cras/cras_input.cc',
        'audio/cras/cras_input.h',
        'audio/cras/cras_unified.cc',
        'audio/cras/cras_unified.h',
      ],
    }],
    ['use_pulseaudio==1', {
      'cflags': [
        '<!@(<(pkg-config) --cflags libpulse)',            # <- this line
      ],
      'defines': [
        'USE_PULSEAUDIO',
      ],
      'conditions': [
        ['linux_link_pulseaudio==0', {
          'defines': [
            'DLOPEN_PULSEAUDIO',
          ],
          'variables': {
            'generate_stubs_script': '../tools/generate_stubs/generate_stubs.py',
            'extra_header': 'audio/pulse/pulse_stub_header.fragment',
            'sig_files': ['audio/pulse/pulse.sigs'],

Solution

See Command-Expansions in the gyp docs

<!@ means Command Expansions

In a command expansion, the entire string contained within the parentheses is passed to the system’s shell. The command’s output is assigned to a string value that may subsequently be expanded in list context in the same way as variable expansions if an @ character is used.

Example:

{
  'sources': [
    '!(echo filename with space.cc)',
  ],
  'libraries': [
    '!@(pkg-config --libs-only-l apr-1)',
  ],
}

will be expand to

{
  'sources': [
    'filename with space.cc',  # no @, expands into a single string
  ],
  'libraries': [  # @ was used, so there's a separate list item for each lib
    '-lapr-1',
    '-lpthread',
  ],
}