build-time: remove 20% more by PCH'ing C++ stdlib

It seems like someone implemented precompiled headers a long time ago
and then it never got ported to meson or maybe didn't work at all.

This is, however, blessedly easy to simply implement. I went looking for
`#define` that could affect the result of precompiling the headers, and
as far as I can tell we aren't doing any of that, so this should truly
just be free build time savings.

Previous state:
Compilation (551 times):
  Parsing (frontend):         1302.1 s
  Codegen & opts (backend):    956.3 s

New state:
**** Time summary:
Compilation (567 times):
  Parsing (frontend):         1123.0 s
  Codegen & opts (backend):   1078.1 s

I wonder if the "regression" in codegen time is just doing the PCH
operation a few times, because meson does it per-target.

Change-Id: I664366b8069bab4851308b3a7571bea97ac64022

misc/pre-commit.nix

@@ -87,11 +87,12 @@ pre-commit-run {
         "file"
         "header"
       ];
-      # generated files; these will never actually be seen by this
-      # check, and are left here as documentation
       excludes = [
-        "(parser|lexer)-tab\\.hh$"
+        ''^src/pch/.*$''
-        "\\.gen\\.hh$"
+        # generated files; these will never actually be seen by this
+        # check, and are left here as documentation
+        ''(parser|lexer)-tab\.hh$''
+        ''\.gen\.hh$''
       ];
       entry = lib.getExe pkgs.check-headers;
     };

package.nix

@@ -169,7 +169,6 @@ stdenv.mkDerivation (finalAttrs: {
           ./boehmgc-coroutine-sp-fallback.diff
           ./doc
           ./misc
-          ./precompiled-headers.h
           ./src
           ./COPYING
         ]

src/libcmd/meson.build

@@ -54,6 +54,7 @@ libcmd = library(
     nlohmann_json,
     lix_doc
   ],
+  cpp_pch : ['../pch/precompiled-headers.hh'],
   install : true,
   # FIXME(Qyriad): is this right?
   install_rpath : libdir,

src/libexpr/meson.build

@@ -145,6 +145,7 @@ libexpr = library(
   include_directories : [
     '../libmain',
   ],
+  cpp_pch : ['../pch/precompiled-headers.hh'],
   install : true,
   # FIXME(Qyriad): is this right?
   install_rpath : libdir,

src/libfetchers/meson.build

@@ -30,6 +30,7 @@ libfetchers = library(
     liblixutil,
     nlohmann_json,
   ],
+  cpp_pch : ['../pch/precompiled-headers.hh'],
   install : true,
   # FIXME(Qyriad): is this right?
   install_rpath : libdir,

src/libmain/meson.build

@@ -20,6 +20,7 @@ libmain = library(
     liblixutil,
     liblixstore,
   ],
+  cpp_pch : ['../pch/precompiled-headers.hh'],
   install : true,
   # FIXME(Qyriad): is this right?
   install_rpath : libdir,

src/libstore/meson.build

@@ -220,6 +220,7 @@ libstore = library(
     nlohmann_json,
   ],
   cpp_args : cpp_args,
+  cpp_pch : ['../pch/precompiled-headers.hh'],
   install : true,
   # FIXME(Qyriad): is this right?
   install_rpath : libdir,

src/libutil/meson.build

@@ -129,6 +129,7 @@ libutil = library(
     openssl,
     nlohmann_json,
   ],
+  cpp_pch : ['../pch/precompiled-headers.hh'],
   implicit_include_directories : true,
   install : true,
 )

src/nix/meson.build

@@ -89,6 +89,7 @@ nix = executable(
     boehm,
     nlohmann_json,
   ],
+  cpp_pch : ['../pch/precompiled-headers.hh'],
   install : true,
   # FIXME(Qyriad): is this right?
   install_rpath : libdir,

tests/unit/meson.build

@@ -68,6 +68,7 @@ libutil_tester = executable(
     liblixutil_test_support,
     nlohmann_json,
   ],
+  cpp_pch : ['../../src/pch/precompiled-headers.hh'],
 )
 
 test(
@@ -102,6 +103,7 @@ libstore_test_support = library(
   include_directories : include_directories(
     'libstore-support',
   ),
+  cpp_pch : ['../../src/pch/precompiled-headers.hh'],
 )
 liblixstore_test_support = declare_dependency(
   include_directories : include_directories('libstore-support'),
@@ -135,6 +137,7 @@ libstore_tester = executable(
     gtest,
     nlohmann_json,
   ],
+  cpp_pch : ['../../src/pch/precompiled-headers.hh'],
 )
 
 test(
@@ -166,6 +169,7 @@ libexpr_test_support = library(
   include_directories : include_directories(
     'libexpr-support',
   ),
+  cpp_pch : ['../../src/pch/precompiled-headers.hh'],
 )
 liblixexpr_test_support = declare_dependency(
   include_directories : include_directories('libexpr-support'),
@@ -199,6 +203,7 @@ libexpr_tester = executable(
     gtest,
     nlohmann_json,
   ],
+  cpp_pch : ['../../src/pch/precompiled-headers.hh'],
 )
 
 test(
@@ -225,6 +230,7 @@ libcmd_tester = executable(
     gtest,
     boost,
   ],
+  cpp_pch : ['../../src/pch/precompiled-headers.hh'],
 )
 
 test(