Clean some header related things.

Revert most of "Hack together a fix for the public headers"

- The `libmain` change is kept, and one more libmain change is made.
  (Need to update Meson and Nix per the package alike).

- The S3 situation is fixed in a different way: the variable is public
  now, used in the header, and fixed accordingly.

- Fix TODO for `HAVE_EMBEDDED_SANDBOX_SHELL`

This reverts commit 2b51250534.

src/libexpr/expr-config.hh

@@ -1,3 +0,0 @@
-// 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

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

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

@@ -10,7 +10,6 @@ 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

@@ -79,11 +79,6 @@ 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(
@@ -168,8 +163,6 @@ 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,
@@ -188,8 +181,4 @@ 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')