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.

auto/os/win32

@@ -9,10 +9,21 @@ CORE_INCS="$WIN32_INCS"
 CORE_DEPS="$WIN32_DEPS"
 CORE_SRCS="$WIN32_SRCS $IOCP_SRCS"
 OS_CONFIG="$WIN32_CONFIG"
-CORE_LIBS="$CORE_LIBS advapi32.lib ws2_32.lib"
 NGX_ICONS="$NGX_WIN32_ICONS"
 SELECT_SRCS=$WIN32_SELECT_SRCS
 
+case "$NGX_CC_NAME" in
+
+    gcc)
+        CORE_LIBS="$CORE_LIBS -ladvapi32 -lws2_32"
+    ;;
+
+    *)
+        CORE_LIBS="$CORE_LIBS advapi32.lib ws2_32.lib"
+    ;;
+
+esac
+
 EVENT_MODULES="$EVENT_MODULES $IOCP_MODULE"
 EVENT_FOUND=YES