pkg-config: always pass --static in static builds

By default, pkg-config output will not include Requires.private
dependencies (which specify dependencies that only apply when building
statically).  It will only do this when passed --static.  In
pkgsStatic, let's ensure pkg-config is always operating in static
mode, because pkgsStatic will ensure that any libraries pkg-config
might find will always be static, and so will always need their
Requires.private dependencies.

This is very useful for Meson builds, because otherwise Meson will
only pass --static to pkg-config if the dependency was explicitly
"static : true", which is not likely to be the case for most stuff
we're building.

An alternative proposal was to patch Meson to add a special
environment variable to force dependencies to be "static : true".  I
feel that the approach I've taken here is less invasive.

pkgs/build-support/pkg-config-wrapper/default.nix

@@ -47,6 +47,9 @@ stdenv.mkDerivation {
   dontBuild = true;
   dontConfigure = true;
 
+  # Additional flags passed to pkg-config.
+  addFlags = lib.optional stdenv.targetPlatform.isStatic "--static";
+
   unpackPhase = ''
     src=$PWD
   '';

pkgs/build-support/pkg-config-wrapper/pkg-config-wrapper.sh

@@ -12,6 +12,8 @@ if [ -z "${NIX_PKG_CONFIG_WRAPPER_FLAGS_SET_@suffixSalt@:-}" ]; then
     source @out@/nix-support/add-flags.sh
 fi
 
+set -- "$@" @addFlags@
+
 if (( ${#role_suffixes[@]} > 0 )); then
     # replace env var with nix-modified one
     PKG_CONFIG_PATH=$PKG_CONFIG_PATH_@suffixSalt@ exec @prog@ "$@"