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!

Makefile

@@ -28,7 +28,7 @@ makefiles = \
   doc/manual/local.mk \
   tests/local.mk
 
-GLOBAL_CXXFLAGS += -std=c++14 -g -Wall
+GLOBAL_CXXFLAGS += -std=c++14 -g -Wall -include config.h
 
 -include Makefile.config
 

src/libexpr/json-to-value.cc

@@ -1,4 +1,3 @@
-#include "config.h"
 #include "json-to-value.hh"
 
 #include <cstring>

src/libexpr/symbol-table.hh

@@ -1,7 +1,5 @@
 #pragma once
 
-#include "config.h"
-
 #include <map>
 #include <unordered_set>
 

src/libexpr/value.hh

@@ -1,6 +1,5 @@
 #pragma once
 
-#include "config.h"
 #include "symbol-table.hh"
 
 #if HAVE_BOEHMGC

src/libmain/shared.cc

@@ -1,5 +1,3 @@
-#include "config.h"
-
 #include "common-args.hh"
 #include "globals.hh"
 #include "shared.hh"

src/libmain/stack.cc

@@ -1,5 +1,3 @@
-#include "config.h"
-
 #include "types.hh"
 
 #include <cstring>

src/libstore/build.cc

@@ -1,5 +1,3 @@
-#include "config.h"
-
 #include "references.hh"
 #include "pathlocks.hh"
 #include "globals.hh"

src/libstore/globals.cc

@@ -1,5 +1,3 @@
-#include "config.h"
-
 #include "globals.hh"
 #include "util.hh"
 #include "archive.hh"

src/libstore/local-store.cc

@@ -1,4 +1,3 @@
-#include "config.h"
 #include "local-store.hh"
 #include "globals.hh"
 #include "archive.hh"

src/libstore/optimise-store.cc

@@ -1,5 +1,3 @@
-#include "config.h"
-
 #include "util.hh"
 #include "local-store.hh"
 #include "globals.hh"

src/libstore/s3-binary-cache-store.cc

@@ -1,5 +1,3 @@
-#include "config.h"
-
 #if ENABLE_S3
 #if __linux__
 

src/libutil/archive.cc

@@ -1,5 +1,3 @@
-#include "config.h"
-
 #include <cerrno>
 #include <algorithm>
 #include <vector>

src/libutil/hash.cc

@@ -1,5 +1,3 @@
-#include "config.h"
-
 #include <iostream>
 #include <cstring>
 

src/libutil/types.hh

@@ -1,6 +1,5 @@
 #pragma once
 
-#include "config.h"
 
 #include "ref.hh"
 

src/libutil/util.cc

@@ -1,5 +1,3 @@
-#include "config.h"
-
 #include "util.hh"
 #include "affinity.hh"
 #include "sync.hh"