meson: Add dependency for for libpkgconf

This allows others using libpkgconf as a dependency to do use this as a
subproject. They can write something like:

```meson
dependency('libpkgconf', fallback : ['libpkgconf', 'dep_libpkgconf'])
```

Then install a wrap file for libpkgconf and build it as part of their
project.

meson.build

@@ -72,6 +72,12 @@ libpkgconf = library('pkgconf',
   soversion : '3',
 )
 
+# For other projects using libpkgconfig as a subproject
+dep_libpkgconf = declare_dependency(
+  link_with : libpkgconf,
+  include_directories : include_directories('.'),
+)
+
 pkg = import('pkgconfig')
 pkg.generate(libpkgconf,
   name : 'libpkgconf',