title: Meson copy/install header files to output directory and keep folder structure
body: <p>Basically i would like to be able to have a mix between the functions install_subdir and install_headers.<br>
  I want to copy all header files from my project source directory over to some other directory and still maintain the subdirectory structure.</p>
  
  <p>Source</p>
  
  <pre><code>MyProject  
  |-- folder1  
  |   |-- file11.h  
  |   |-- file11.cpp  
  |   |-- file12.h  
  |   `-- file12.cpp  
  `-- folder2  
      `-- file21.h
  </code></pre>
  
  <p>Destination</p>
  
  <pre><code>
  MyProject
  |-- folder1
  |   |-- file11.h
  |   |-- file12.h
  `-- folder2
      `-- file21.h
  </code></pre>
  
  <p>What I tried was copying the source directory and exclude all cpp files and to just use the intended <code>install_headers()</code> function but both didn't work out.<br>
  I added comments to things I did and why. Maybe somebody knows what's up:</p>
  
  <pre><code>project('MyProject', 'cpp')
  
  test_src = [  'src/MyProject/folder1/file11.cpp'
                'src/MyProject/folder1/file12.cpp']
  
  # Passing files seems to be preferred but exclude_files only takes list of strings
  # test_src = files([  'src/MyProject/folder1/file11.cpp'
  #                     'src/MyProject/folder1/file12.cpp'])
  
  test_hdr = files([  'src/MyProject/folder1/file11.h',
                      'src/MyProject/folder1/file12.h',
                      'src/MyProject/folder2/file21.h'])
  
  
  base_dir = meson.current_source_dir()
  
  static_library('MyProject', name_prefix: '', name_suffix : 'lib', 
                  sources: test_src,
                  install: true,
                  install_dir: base_dir + '/build/lib')
  
  # Produces flat hierarchy       
  install_headers(test_hdr, install_dir: base_dir + '/build/include')
  
  # Also copies all cpp files into destination folder
  install_subdir('src/MyProject', install_dir: base_dir + '/build/include', exclude_files: '*.cpp')
  
  # Same result as wildcard exclusion
  install_subdir('src/MyProject', install_dir: base_dir + '/build/include', exclude_files: test_src)
  </code></pre>
  
  <p>Does anyone have a solution to this?<br>
  I have a full list of my sources and headers if that is necessary for any method.<br>
  I am currently copying the files via shell commands, but including this into the install/build process would be preferred.</p>
  
  <p><strong>// EDIT:</strong><br>
  I am using meson-build on windows.</p>