meson: Disable PCH for GCC

GCC doesn't really benefit as much as Clang does from
using precompiled headers. Another aspect to consider is that
clangd doesn't really like GCC's PCH flags in the compilation database,
so GCC based devshells would continue to work with clangd.

This also has the slight advantage of ensuring that our includes are in
order, since we build with both Clang and GCC.

nix-meson-build-support/common/meson.build

@@ -19,6 +19,9 @@ add_project_arguments(
   language : 'cpp',
 )
 
+# GCC doesn't benefit much from precompiled headers.
+do_pch = cxx.get_id() == 'clang'
+
 # This is a clang-only option for improving build times.
 # It forces the instantiation of templates in the PCH itself and
 # not every translation unit it's included in.

src/libcmd/meson.build

@@ -92,7 +92,7 @@ this_library = library(
   link_args: linker_export_flags,
   prelink : true, # For C++ static initializers
   install : true,
-  cpp_pch : ['pch/precompiled-headers.hh']
+  cpp_pch : do_pch ? ['pch/precompiled-headers.hh'] : []
 )
 
 install_headers(headers, subdir : 'nix/cmd', preserve_path : true)

src/libexpr/meson.build

@@ -178,7 +178,7 @@ this_library = library(
   link_args: linker_export_flags,
   prelink : true, # For C++ static initializers
   install : true,
-  cpp_pch : ['pch/precompiled-headers.hh']
+  cpp_pch : do_pch ? ['pch/precompiled-headers.hh'] : []
 )
 
 install_headers(headers, subdir : 'nix/expr', preserve_path : true)

src/libstore/meson.build

@@ -350,7 +350,7 @@ this_library = library(
   link_args: linker_export_flags,
   prelink : true, # For C++ static initializers
   install : true,
-  cpp_pch : ['pch/precompiled-headers.hh']
+  cpp_pch : do_pch ? ['pch/precompiled-headers.hh'] : []
 )
 
 install_headers(headers, subdir : 'nix/store', preserve_path : true)

src/libutil/meson.build

@@ -191,7 +191,7 @@ this_library = library(
   link_args: linker_export_flags,
   prelink : true, # For C++ static initializers
   install : true,
-  cpp_pch : 'pch/precompiled-headers.hh'
+  cpp_pch : do_pch ? ['pch/precompiled-headers.hh'] : []
 )
 
 install_headers(headers, subdir : 'nix/util', preserve_path : true)

src/nix/meson.build

@@ -186,7 +186,7 @@ this_exe = executable(
   include_directories : include_dirs,
   link_args: linker_export_flags,
   install : true,
-  cpp_pch : ['pch/precompiled-headers.hh']
+  cpp_pch : do_pch ? ['pch/precompiled-headers.hh'] : []
 )
 
 meson.override_find_program('nix', this_exe)