Win32: MinGW GCC compatibility.

Several warnings silenced, notably (ngx_socket_t) -1 is now checked
on socket operations instead of -1, as ngx_socket_t is unsigned on win32
and gcc complains on comparison.

With this patch, it's now possible to compile nginx using mingw gcc,
with options we normally compile on win32.

src/core/ngx_connection.c

diff --git a/src/core/ngx_connection.c b/src/core/ngx_connection.c
index e12d3ef..28bf6ba 100644
--- a/src/core/ngx_connection.c
+++ b/src/core/ngx_connection.c
@@ -297,7 +297,7 @@
                 continue;
             }
 
-            if (ls[i].fd != -1) {
+            if (ls[i].fd != (ngx_socket_t) -1) {
                 continue;
             }
 
@@ -312,7 +312,7 @@
 
             s = ngx_socket(ls[i].sockaddr->sa_family, ls[i].type, 0);
 
-            if (s == -1) {
+            if (s == (ngx_socket_t) -1) {
                 ngx_log_error(NGX_LOG_EMERG, log, ngx_socket_errno,
                               ngx_socket_n " %V failed", &ls[i].addr_text);
                 return NGX_ERROR;
@@ -863,7 +863,7 @@
     ngx_uint_t    log_error, level;
     ngx_socket_t  fd;
 
-    if (c->fd == -1) {
+    if (c->fd == (ngx_socket_t) -1) {
         ngx_log_error(NGX_LOG_ALERT, c->log, 0, "connection already closed");
         return;
     }

src/core/ngx_cycle.c

diff --git a/src/core/ngx_cycle.c b/src/core/ngx_cycle.c
index 2c006bc..eb39ab2 100644
--- a/src/core/ngx_cycle.c
+++ b/src/core/ngx_cycle.c
@@ -543,7 +543,7 @@
                 }
             }
 
-            if (nls[n].fd == -1) {
+            if (nls[n].fd == (ngx_socket_t) -1) {
                 nls[n].open = 1;
             }
         }
@@ -649,7 +649,7 @@
     ls = old_cycle->listening.elts;
     for (i = 0; i < old_cycle->listening.nelts; i++) {
 
-        if (ls[i].remain || ls[i].fd == -1) {
+        if (ls[i].remain || ls[i].fd == (ngx_socket_t) -1) {
             continue;
         }
 
@@ -813,7 +813,7 @@
 
     ls = cycle->listening.elts;
     for (i = 0; i < cycle->listening.nelts; i++) {
-        if (ls[i].fd == -1 || !ls[i].open) {
+        if (ls[i].fd == (ngx_socket_t) -1 || !ls[i].open) {
             continue;
         }
 

src/core/ngx_resolver.c

diff --git a/src/core/ngx_resolver.c b/src/core/ngx_resolver.c
index d59d0c4..d94477a 100644
--- a/src/core/ngx_resolver.c
+++ b/src/core/ngx_resolver.c
@@ -2221,7 +2221,7 @@
 
     ngx_log_debug1(NGX_LOG_DEBUG_EVENT, &uc->log, 0, "UDP socket %d", s);
 
-    if (s == -1) {
+    if (s == (ngx_socket_t) -1) {
         ngx_log_error(NGX_LOG_ALERT, &uc->log, ngx_socket_errno,
                       ngx_socket_n " failed");
         return NGX_ERROR;