validatePkgConfig: init

This adds the `validatePkgConfig` hook, which can be used to validate
pkg-config files in the output(s). Currently, this will just run
`pkg-config --validate` on all `.pc` files, capturing errors such as
the issue that was fixed in #87789.

The hook could be extended in the future with more fine-grained
checks.

pkgs/build-support/setup-hooks/validate-pkg-config.sh

@@ -0,0 +1,19 @@
+# This setup hook validates each pkgconfig file in each output.
+
+fixupOutputHooks+=(_validatePkgConfig)
+
+_validatePkgConfig() {
+    for pc in $(find "$prefix" -name '*.pc'); do
+        local bail=0
+
+        # Do not fail immediately. It's nice to see all errors when
+        # there are multiple pkgconfig files.
+        if ! pkg-config --validate "$pc"; then
+            bail=1
+        fi
+    done
+
+    if [ $bail -eq 1 ]; then
+        exit 1
+    fi
+}