meson: install missing/generated headers

one headers (args/root.hh) was simply missing, and the generated headers
were not installed. not all of them *should* be installed either, only a
select few (and sadly this needs a custom target for each one, it seems)

Change-Id: I37b25517895d0e5e521abc1202fa65624de57ed1

src/libexpr/flake/meson.build

@@ -0,0 +1,8 @@
+libexpr_generated_headers += custom_target(
+  command : [ 'bash', '-c', 'echo \'R"__NIX_STR(\' | cat - @INPUT@ && echo \')__NIX_STR"\'' ],
+  input : 'call-flake.nix',
+  output : '@PLAINNAME@.gen.hh',
+  capture : true,
+  install : true,
+  install_dir : includedir / 'nix/flake',
+)

src/libexpr/meson.build

@@ -49,10 +49,20 @@ meson.add_install_script(
   '@0@'.format(includedir),
 )
 
-imported_drv_to_derivation_gen = gen_header.process('imported-drv-to-derivation.nix')
-fetchurl_gen = gen_header.process('fetchurl.nix')
-derivation_gen = gen_header.process('primops/derivation.nix', preserve_path_from : meson.current_source_dir())
-call_flake_gen = gen_header.process('flake/call-flake.nix')
+libexpr_generated_headers = [
+  gen_header.process('primops/derivation.nix', preserve_path_from : meson.current_source_dir()),
+]
+foreach header : [ 'imported-drv-to-derivation.nix', 'fetchurl.nix' ]
+  libexpr_generated_headers += custom_target(
+    command : [ 'bash', '-c', 'echo \'R"__NIX_STR(\' | cat - @INPUT@ && echo \')__NIX_STR"\'' ],
+    input : header,
+    output : '@PLAINNAME@.gen.hh',
+    capture : true,
+    install : true,
+    install_dir : includedir / 'nix',
+  )
+endforeach
+subdir('flake')
 
 libexpr_sources = files(
   'attr-path.cc',
@@ -121,10 +131,7 @@ libexpr = library(
   libexpr_sources,
   parser_tab,
   lexer_tab,
-  imported_drv_to_derivation_gen,
-  fetchurl_gen,
-  derivation_gen,
-  call_flake_gen,
+  libexpr_generated_headers,
   dependencies : [
     liblixutil,
     liblixstore,