Improve toolchain detection for WinCE cross compilation:

When cross compiling WinCE with the arm-wince-cegcc-gcc C compiler
symbol __CEGCC__ is defined and the unix-like compatibility layer
is used. For our purposes this is not a native Windows build.

When cross compiling WinCE with the arm-wince-mingw32ce-gcc C compiler
symbol __MINGW32CE__ is defined and the unix-like compatibility layer
is not used. For our purposes this _is_ a native Windows build.

acinclude.m4

@@ -14,7 +14,7 @@ AC_DEFUN([CURL_CHECK_HEADER_WINDOWS], [
 #endif
 #include <windows.h>
       ],[
-#ifdef __CYGWIN__
+#if defined(__CYGWIN__) || defined(__CEGCC__)
         HAVE_WINDOWS_H shall not be defined.
 #else
         int dummy=2*WINVER;
@@ -50,7 +50,7 @@ AC_DEFUN([CURL_CHECK_NATIVE_WINDOWS], [
       AC_COMPILE_IFELSE([
         AC_LANG_PROGRAM([
         ],[
-#ifdef __MINGW32__
+#if defined(__MINGW32__) || defined(__MINGW32CE__)
           int dummy=1;
 #else
           Not a native Windows build target.
@@ -88,7 +88,7 @@ AC_DEFUN([CURL_CHECK_HEADER_WINSOCK], [
 #include <windows.h>
 #include <winsock.h>
       ],[
-#ifdef __CYGWIN__
+#if defined(__CYGWIN__) || defined(__CEGCC__)
         HAVE_WINSOCK_H shall not be defined.
 #else
         int dummy=WSACleanup();
@@ -125,7 +125,7 @@ AC_DEFUN([CURL_CHECK_HEADER_WINSOCK2], [
 #include <windows.h>
 #include <winsock2.h>
       ],[
-#if defined(__CYGWIN__) || defined(_WIN32_WCE)
+#if defined(__CYGWIN__) || defined(__CEGCC__) || defined(__MINGW32CE__)
         HAVE_WINSOCK2_H shall not be defined.
 #else
         int dummy=2*IPPROTO_ESP;
@@ -163,7 +163,7 @@ AC_DEFUN([CURL_CHECK_HEADER_WS2TCPIP], [
 #include <winsock2.h>
 #include <ws2tcpip.h>
       ],[
-#if defined(__CYGWIN__) || defined(_WIN32_WCE)
+#if defined(__CYGWIN__) || defined(__CEGCC__) || defined(__MINGW32CE__)
         HAVE_WS2TCPIP_H shall not be defined.
 #else
         int dummy=2*IP_PKTINFO;