Unexpose config headers (low hanging fruit only)

- Some headers were completely redundant and have been removed.
- Other headers have been turned private.
- Unnecessary meson.build code has been removed.
- libutil-tests now has a private config header, where previously
  it had none. This removes the need to expose a package version
  macro publicly.

(cherry picked from commit b86a76044e)

src/libutil-c/meson.build

@@ -23,7 +23,6 @@ deps_public_maybe_subproject = [
 ]
 subdir('nix-meson-build-support/subprojects')
 
-# TODO rename, because it will conflict with downstream projects
 configdata.set_quoted('PACKAGE_VERSION', meson.project_version())
 
 config_h = configure_file(
@@ -38,8 +37,6 @@ add_project_arguments(
   # From C++ libraries, only for internals
   '-include', 'config-util.hh',
 
-  # From C libraries, for our public, installed headers too
-  '-include', 'config-util.h',
   language : 'cpp',
 )
 
@@ -51,7 +48,7 @@ sources = files(
 
 include_dirs = [include_directories('.')]
 
-headers = [config_h] + files(
+headers = files(
   'nix_api_util.h',
 )
 
@@ -64,6 +61,7 @@ subdir('nix-meson-build-support/windows-version')
 this_library = library(
   'nixutilc',
   sources,
+  config_h,
   dependencies : deps_public + deps_private + deps_other,
   include_directories : include_dirs,
   link_args: linker_export_flags,

src/libutil-c/nix_api_util.cc

@@ -7,6 +7,8 @@
 #include <cxxabi.h>
 #include <typeinfo>
 
+#include "config-util.h"
+
 nix_c_context * nix_c_context_create()
 {
     return new nix_c_context();