Hack together a fix for the public headers

Please fix this.

src/libexpr/expr-config.hh

@@ -0,0 +1,3 @@
+// TODO: Remove this damn file while keeping public config headers working
+#error \
+    "This file is a placeholder. It only exists so that meson accepts the symbolic link include/nix/expr/config.hh to this file, but we expect meson to overwrite it with the real file. Apparently that did not happen. I deeply apologize for this mess."

src/libexpr/include/nix/expr/config.hh

@@ -0,0 +1 @@
+../../../expr-config.hh

src/libexpr/include/nix/expr/meson.build

@@ -10,6 +10,7 @@ config_pub_h = configure_file(
 headers = [config_pub_h] + files(
   'attr-path.hh',
   'attr-set.hh',
+  'config.hh',
   'eval-cache.hh',
   'eval-error.hh',
   'eval-gc.hh',

src/libexpr/meson.build

@@ -79,6 +79,11 @@ config_priv_h = configure_file(
   output : 'expr-config-private.hh',
 )
 
+config_pub_h = configure_file(
+  configuration : configdata_pub,
+  output : 'expr-config.hh',
+)
+
 subdir('nix-meson-build-support/common')
 
 parser_tab = custom_target(
@@ -163,6 +168,8 @@ subdir('primops')
 subdir('nix-meson-build-support/export-all-symbols')
 subdir('nix-meson-build-support/windows-version')
 
+headers += [config_pub_h]
+
 this_library = library(
   'nixexpr',
   sources,
@@ -181,4 +188,8 @@ install_headers(headers, subdir : 'nix/expr', preserve_path : true)
 
 libraries_private = []
 
+nixexpr_dep = declare_dependency(
+  include_directories : include_directories('.'),
+  link_with : this_library,
+)
 subdir('nix-meson-build-support/export')