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!

5 files changed, 0 insertions, 9 deletions

diff --git a/src/libstore/build.cc b/src/libstore/build.cc
index 5d6fff4e3..1aee150fd 100644
--- a/src/libstore/build.cc
+++ b/src/libstore/build.cc
@@ -1,5 +1,3 @@
-#include "config.h"
-
 #include "references.hh"
 #include "pathlocks.hh"
 #include "globals.hh"
diff --git a/src/libstore/globals.cc b/src/libstore/globals.cc
index 00b468892..90f83a5bb 100644
--- a/src/libstore/globals.cc
+++ b/src/libstore/globals.cc
@@ -1,5 +1,3 @@
-#include "config.h"
-
 #include "globals.hh"
 #include "util.hh"
 #include "archive.hh"
diff --git a/src/libstore/local-store.cc b/src/libstore/local-store.cc
index 612efde7b..4c161cfb3 100644
--- a/src/libstore/local-store.cc
+++ b/src/libstore/local-store.cc
@@ -1,4 +1,3 @@
-#include "config.h"
 #include "local-store.hh"
 #include "globals.hh"
 #include "archive.hh"
diff --git a/src/libstore/optimise-store.cc b/src/libstore/optimise-store.cc
index b71c7e905..cf234e35d 100644
--- a/src/libstore/optimise-store.cc
+++ b/src/libstore/optimise-store.cc
@@ -1,5 +1,3 @@
-#include "config.h"
-
 #include "util.hh"
 #include "local-store.hh"
 #include "globals.hh"
diff --git a/src/libstore/s3-binary-cache-store.cc b/src/libstore/s3-binary-cache-store.cc
index ccb71f1ee..cc1b33104 100644
--- a/src/libstore/s3-binary-cache-store.cc
+++ b/src/libstore/s3-binary-cache-store.cc
@@ -1,5 +1,3 @@
-#include "config.h"
-
 #if ENABLE_S3
 #if __linux__