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/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')