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, 35 insertions, 0 deletions

diff --git a/docs/markdown/snippets/install_headers_preserve_path_arg.md b/docs/markdown/snippets/install_headers_preserve_path_arg.md
new file mode 100644
index 0000000..c41fceb
--- /dev/null
+++ b/docs/markdown/snippets/install_headers_preserve_path_arg.md
@@ -0,0 +1,35 @@
+## Added preserve_path arg to 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
+
+```meson
+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
+```