build: add datadir to pkgconfig files

Mutter wants sysprof's exact dbus interface directory during build time
* https://gitlab.gnome.org/GNOME/mutter/blob/master/src/meson.build#L714

On certain platforms mutter's datadir != sysprof's datadir.
This is initial work for a forthcoming PR to do
```
sysprof_dep.get_pkgconfig_variable('datadir')
```
in mutter for compatibility with these platforms.

This patch is on behalf of nixpkgs where we actually have to carry a 
downstream patch [0] for this exact reason. We install everything into 
their own immutable prefix under `/nix/store`.

[0]: a821167046/pkgs/desktops/gnome-3/core/mutter/0001-build-use-get_pkgconfig_variable-for-sysprof-dbusdir.patch

meson.build

@@ -15,6 +15,7 @@ i18n = import('i18n')
 libsysprof_api_version = 3
 version_split = meson.project_version().split('.')
 datadir = get_option('datadir')
+datadir_for_pc_file = join_paths('${prefix}', datadir)
 podir = join_paths(meson.source_root(), 'po')
 
 glib_req_version = '>= 2.61.3'

src/libsysprof-capture/meson.build

@@ -64,4 +64,7 @@ pkgconfig.generate(
   description: 'The static capture library for tools that generate profiling capture data',
   install_dir: join_paths(get_option('libdir'), 'pkgconfig'),
      requires: [ 'glib-2.0' ],
+     variables: [
+       'datadir=' + datadir_for_pc_file,
+     ],
 )

src/libsysprof-ui/meson.build

@@ -114,6 +114,9 @@ pkgconfig.generate(
   description: 'The UI library for GTK applications embedding sysprof',
   install_dir: join_paths(get_option('libdir'), 'pkgconfig'),
      requires: [ 'gio-2.0', 'gtk+-3.0' ],
+     variables: [
+       'datadir=' + datadir_for_pc_file,
+     ],
 )
 
 install_headers(libsysprof_ui_public_headers, subdir: sysprof_header_subdir)

src/libsysprof/meson.build

@@ -138,6 +138,9 @@ pkgconfig.generate(
   description: 'The library for console applications embedding sysprof',
   install_dir: join_paths(get_option('libdir'), 'pkgconfig'),
      requires: [ 'gio-2.0' ],
+     variables: [
+       'datadir=' + datadir_for_pc_file,
+     ],
 )
 
 install_headers(libsysprof_public_headers, subdir: sysprof_header_subdir)