meson: don't strip subdirs from header install process

Apparently meson doesn't really support this cleanly, and also somehow
doesn't have helper functions for getting the basename of paths etc., so
we have to implement this in our own logic. Somewhat ugly but okay,
whatever.

Fixes https://github.com/haasn/libplacebo/issues/73

src/meson.build

@@ -287,7 +287,15 @@ lib = library('placebo', sources,
 
 # Install process
 foreach h : headers
-  install_headers('include' / proj_name / h, subdir: proj_name)
+  parts = h.split('/')
+  path = proj_name
+  foreach p : parts
+    if p != parts[-1]
+      path = path / p
+    endif
+  endforeach
+
+  install_headers('include' / proj_name / h, subdir: path)
 endforeach
 
 pkg = import('pkgconfig')