pkgconfig module: allow custom variables to reference builtin directories

Automatically generate additional variables and write them into the
generated pkg-config file.

This means projects no longer need to manually define the ones they
use, which is annoying for dataonly usages (it used to forbid setting
the base library-relevant "reserved" ones, and now allows it only for
dataonly. But it's bloat to manualy list them anyway).

It also fixes a regression in commit
248e6cf4736ef9ec636228da66c28f9be03aa74f which caused libdir to not be
set, and to be unsettable, if the pkg-config file has no libraries but
uses the ${libdir} expansion in a custom variable. This could be
considered likely a case for dataonly, but it's not guaranteed.

1 files changed, 31 insertions, 0 deletions

diff --git a/docs/markdown/snippets/pkgconfig_directory_variables.md b/docs/markdown/snippets/pkgconfig_directory_variables.md
new file mode 100644
index 0000000..27a4069
--- /dev/null
+++ b/docs/markdown/snippets/pkgconfig_directory_variables.md
@@ -0,0 +1,31 @@
+## pkgconfig.generate will now include variables for builtin directories when referenced
+
+When using the `variables:` family of kwargs to `pkgconfig.generate` to refer
+to installed paths, traditionally only `prefix`, `includedir`, and `libdir`
+were available by default, and generating a correct (relocatable) pkg-config
+file required manually constructing variables for e.g. `datadir`.
+
+Meson now checks each variable to see if it begins with a reference to a
+standard directory, and if so, adds it to the list of directories for which a
+builtin variable is created.
+
+For example, before it was necessary to do this:
+```meson
+pkgconfig.generate(
+    name: 'bash-completion',
+    description: 'programmable completion for the bash shell',
+    dataonly: true,
+    variables: {
+        'prefix': get_option('prefix'),
+        'datadir': join_paths('${prefix}', get_option('datadir')),
+        'sysconfdir': join_paths('${prefix}', get_option('sysconfdir')),
+
+        'compatdir': '${sysconfdir}/bash_completion.d',
+        'completionsdir': '${datadir}/bash-completion/completions',
+        'helpersdir': '${datadir}/bash-completion/helpers',
+    },
+    install_dir: join_paths(get_option('datadir'), 'pkgconfig'),
+)
+```
+
+Now the first three variables are not needed.