Include config.h implicitly with '-include config.h' in CFLAGS

Because config.h can #define things like _FILE_OFFSET_BITS=64 and not
every compilation unit includes config.h, we currently compile half of
Nix with _FILE_OFFSET_BITS=64 and other half with _FILE_OFFSET_BITS
unset. This causes major havoc with the Settings class on e.g. 32-bit ARM,
where different compilation units disagree with the struct layout.

E.g.:

diff --git a/src/libstore/globals.cc b/src/libstore/globals.cc
@@ -166,6 +166,8 @@ void Settings::update()
     _get(useSubstitutes, "build-use-substitutes");
+    fprintf(stderr, "at Settings::update(): &useSubstitutes = %p\n", &nix::settings.useSubstitutes);
     _get(buildUsersGroup, "build-users-group");
diff --git a/src/libstore/remote-store.cc b/src/libstore/remote-store.cc
+++ b/src/libstore/remote-store.cc
@@ -138,6 +138,8 @@ void RemoteStore::initConnection(Connection & conn)
 void RemoteStore::setOptions(Connection & conn)
 {
+    fprintf(stderr, "at RemoteStore::setOptions(): &useSubstitutes = %p\n", &nix::settings.useSubstitutes);
     conn.to << wopSetOptions

Gave me:

at Settings::update(): &useSubstitutes = 0xb6e5c5cb
at RemoteStore::setOptions(): &useSubstitutes = 0xb6e5c5c7

That was not a fun one to debug!

3 files changed, 0 insertions, 4 deletions

diff --git a/src/libexpr/json-to-value.cc b/src/libexpr/json-to-value.cc
index f671802bcc24..c189cdef35e7 100644
--- a/src/libexpr/json-to-value.cc
+++ b/src/libexpr/json-to-value.cc
@@ -1,4 +1,3 @@
-#include "config.h"
 #include "json-to-value.hh"
 
 #include <cstring>
diff --git a/src/libexpr/symbol-table.hh b/src/libexpr/symbol-table.hh
index 2fdf820211c8..c2ee49dd32fb 100644
--- a/src/libexpr/symbol-table.hh
+++ b/src/libexpr/symbol-table.hh
@@ -1,7 +1,5 @@
 #pragma once
 
-#include "config.h"
-
 #include <map>
 #include <unordered_set>
 
diff --git a/src/libexpr/value.hh b/src/libexpr/value.hh
index 271e6a1b24a2..81f918d48de7 100644
--- a/src/libexpr/value.hh
+++ b/src/libexpr/value.hh
@@ -1,6 +1,5 @@
 #pragma once
 
-#include "config.h"
 #include "symbol-table.hh"
 
 #if HAVE_BOEHMGC