oslib-win32: only provide localtime_r/gmtime_r if missing

The oslib-win32 file currently provides a localtime_r and
gmtime_r replacement unconditionally. Some versions of
Mingw-w64 would provide crude macros for localtime_r/gmtime_r
which QEMU takes care to disable. Latest versions of Mingw-w64
now provide actual functions for localtime_r/gmtime_r, but
with a twist that you have to include unistd.h or pthread.h
before including time.h.  By luck some files in QEMU have
such an include order, resulting in compile errors:

  CC    util/osdep.o
In file included from include/qemu-common.h:48:0,
                 from util/osdep.c:48:
include/sysemu/os-win32.h:77:12: error: redundant redeclaration of 'gmtime_r' [-Werror=redundant-decls]
 struct tm *gmtime_r(const time_t *timep, struct tm *result);
            ^
In file included from include/qemu-common.h:35:0,
                 from util/osdep.c:48:
/usr/i686-w64-mingw32/sys-root/mingw/include/time.h:272:107: note: previous definition of 'gmtime_r' was here
In file included from include/qemu-common.h:48:0,
                 from util/osdep.c:48:
include/sysemu/os-win32.h:79:12: error: redundant redeclaration of 'localtime_r' [-Werror=redundant-decls]
 struct tm *localtime_r(const time_t *timep, struct tm *result);
            ^
In file included from include/qemu-common.h:35:0,
                 from util/osdep.c:48:
/usr/i686-w64-mingw32/sys-root/mingw/include/time.h:269:107: note: previous definition of 'localtime_r' was here

This change adds a configure test to see if localtime_r
exits, and only enables the QEMU impl if missing. We also
re-arrange qemu-common.h try attempt to guarantee that all
source files get unistd.h before time.h and thus see the
localtime_r/gmtime_r defs.

[sw: Use "official" spellings for Mingw-w64, MinGW in comments.]
[sw: Terminate sentences with a dot in comments.]

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Reviewed-by: Denis V. Lunev <den@openvz.org>
Signed-off-by: Stefan Weil <sw@weilnetz.de>

configure

@@ -1736,6 +1736,37 @@ else
   l2tpv3=no
 fi
 
+##########################################
+# MinGW / Mingw-w64 localtime_r/gmtime_r check
+
+if test "$mingw32" = "yes"; then
+    # Some versions of MinGW / Mingw-w64 lack localtime_r
+    # and gmtime_r entirely.
+    #
+    # Some versions of Mingw-w64 define a macro for
+    # localtime_r/gmtime_r.
+    #
+    # Some versions of Mingw-w64 will define functions
+    # for localtime_r/gmtime_r, but only if you have
+    # _POSIX_THREAD_SAFE_FUNCTIONS defined. For fun
+    # though, unistd.h and pthread.h both define
+    # that for you.
+    #
+    # So this #undef localtime_r and #include <unistd.h>
+    # are not in fact redundant.
+cat > $TMPC << EOF
+#include <unistd.h>
+#include <time.h>
+#undef localtime_r
+int main(void) { localtime_r(NULL, NULL); return 0; }
+EOF
+    if compile_prog "" "" ; then
+        localtime_r="yes"
+    else
+        localtime_r="no"
+    fi
+fi
+
 ##########################################
 # pkg-config probe
 
@@ -5034,6 +5065,9 @@ fi
 if test "$zero_malloc" = "yes" ; then
   echo "CONFIG_ZERO_MALLOC=y" >> $config_host_mak
 fi
+if test "$localtime_r" = "yes" ; then
+  echo "CONFIG_LOCALTIME_R=y" >> $config_host_mak
+fi
 if test "$qom_cast_debug" = "yes" ; then
   echo "CONFIG_QOM_CAST_DEBUG=y" >> $config_host_mak
 fi

include/qemu/osdep.h

@@ -38,10 +38,12 @@
 #include <strings.h>
 #include <inttypes.h>
 #include <limits.h>
+/* Put unistd.h before time.h as that triggers localtime_r/gmtime_r
+ * function availability on recentish Mingw-w64 platforms. */
+#include <unistd.h>
 #include <time.h>
 #include <ctype.h>
 #include <errno.h>
-#include <unistd.h>
 #include <fcntl.h>
 #include <sys/stat.h>
 #include <sys/time.h>

include/sysemu/os-win32.h

@@ -73,10 +73,12 @@
 #define siglongjmp(env, val) longjmp(env, val)
 
 /* Missing POSIX functions. Don't use MinGW-w64 macros. */
+#ifndef CONFIG_LOCALTIME_R
 #undef gmtime_r
 struct tm *gmtime_r(const time_t *timep, struct tm *result);
 #undef localtime_r
 struct tm *localtime_r(const time_t *timep, struct tm *result);
+#endif /* CONFIG_LOCALTIME_R */
 
 
 static inline void os_setup_signal_handling(void) {}

util/oslib-win32.c

@@ -95,6 +95,7 @@ void qemu_anon_ram_free(void *ptr, size_t size)
     }
 }
 
+#ifndef CONFIG_LOCALTIME_R
 /* FIXME: add proper locking */
 struct tm *gmtime_r(const time_t *timep, struct tm *result)
 {
@@ -118,6 +119,7 @@ struct tm *localtime_r(const time_t *timep, struct tm *result)
     }
     return p;
 }
+#endif /* CONFIG_LOCALTIME_R */
 
 void qemu_set_block(int fd)
 {