Implement `preserve_path` for install_headers

The `install_headers` function now has an optional argument
`preserve_path` that allows installing multi-directory
headerfile structures that live alongside sourcecode with a
single command.

For example, the headerfile structure

headers = [
  'one.h',
  'two.h',
  'alpha/one.h',
  'alpha/two.h',
  'alpha/three.h'
  'beta/one.h'
]

can now be passed to `install_headers(headers, subdir: 'mylib', preserve_path: true)`
and the resulting directory tree will look like

{prefix}
└── include
    └── mylib
        ├── alpha
        │   ├── one.h
        │   ├── two.h
        │   └── three.h
        ├── beta
        │   └── one.h
        ├── one.h
        └── two.h

Fixes #3371

1 files changed, 4 insertions, 3 deletions

diff --git a/test cases/common/9 header install/meson.build b/test cases/common/9 header install/meson.build
index 833b4d4..8c83be6 100644
--- a/test cases/common/9 header install/meson.build
+++ b/test cases/common/9 header install/meson.build
@@ -1,11 +1,12 @@
 project('header install')
 
-as_array = ['subdir.h']
+as_array = ['subdir.h', 'child/childdir.h']
 
 subdir('vanishing_subdir')
 subdir('sub')
 
 h1 = install_headers('rootdir.h')
 h2 = install_headers(as_array, subdir : 'subdir')
-h3 = install_headers(subheader)
-h4 = install_headers(disabler())
+h3 = install_headers(as_array, subdir : 'subdir', preserve_path : true)
+h4 = install_headers(subheader)
+h5 = install_headers(disabler())