Add -Wundef to make #if FOO an error if not defined

This commit has all the straightforward stuff.

src/libutil/archive.cc

@@ -17,7 +17,7 @@ namespace nix {
 struct ArchiveSettings : Config
 {
     Setting<bool> useCaseHack{this,
-        #if __APPLE__
+        #ifdef __APPLE__
             true,
         #else
             false,

src/libutil/file-descriptor.cc

@@ -98,7 +98,7 @@ void AutoCloseFD::fsync() const
         result =
 #ifdef _WIN32
             ::FlushFileBuffers(fd)
-#elif __APPLE__
+#elif defined(__APPLE__)
             ::fcntl(fd, F_FULLFSYNC)
 #else
             ::fsync(fd)

src/libutil/file-system.cc

@@ -576,7 +576,7 @@ Path createTempDir(const Path & tmpRoot, const Path & prefix,
                     , mode
 #endif
                     ) == 0) {
-#if __FreeBSD__
+#ifdef __FreeBSD__
             /* Explicitly set the group of the directory.  This is to
                work around around problems caused by BSD's group
                ownership semantics (directories inherit the group of

src/libutil/fs-sink.cc

@@ -4,7 +4,7 @@
 #include "nix/util/config-global.hh"
 #include "nix/util/fs-sink.hh"
 
-#if _WIN32
+#ifdef _WIN32
 # include <fileapi.h>
 # include "nix/util/file-path.hh"
 # include "nix/util/windows-error.hh"

src/libutil/include/nix/util/file-descriptor.hh

@@ -18,7 +18,7 @@ struct Source;
  * Operating System capability
  */
 using Descriptor =
-#if _WIN32
+#ifdef _WIN32
     HANDLE
 #else
     int
@@ -26,7 +26,7 @@ using Descriptor =
     ;
 
 const Descriptor INVALID_DESCRIPTOR =
-#if _WIN32
+#ifdef _WIN32
     INVALID_HANDLE_VALUE
 #else
     -1

src/libutil/terminal.cc

@@ -2,7 +2,7 @@
 #include "nix/util/environment-variables.hh"
 #include "nix/util/sync.hh"
 
-#if _WIN32
+#ifdef _WIN32
 # include <io.h>
 # define WIN32_LEAN_AND_MEAN
 # include <windows.h>

src/libutil/unix/processes.cc

@@ -78,7 +78,7 @@ int Pid::kill()
         /* On BSDs, killing a process group will return EPERM if all
            processes in the group are zombies (or something like
            that). So try to detect and ignore that situation. */
-#if __FreeBSD__ || __APPLE__
+#if defined(__FreeBSD__) || defined(__APPLE__)
         if (errno != EPERM || ::kill(pid, 0) != 0)
 #endif
             logError(SysError("killing process %d", pid).info());