configure: Correctly detect if we are cross-compiling for win32

First, it's $host_os which should be checked, not $target (remember,
'build' is where we build, host is where the program will be run, and
target is only for compiler - for which system the compiler will
generate code.

Second, correct OS pattern - e.g. on debian with i586-mingw32msvc-gcc,
host_os is mingw32msvc, so we better for *mingw32*.

Also, it's better to call AC_CANONICAL_* for normalizing variables.

Build tested with `configure --host=i586-mingw32msvc` on Debian
GNU/Linux 5.0 (Lenny).

Cc: Александр Сёмуха <sav@mns.spb.ru>
Signed-off-by: Kirill Smelkov <kirr@mns.spb.ru>
Signed-off-by: Stéphane Raimbault <stephane.raimbault@gmail.com>

configure.ac

@@ -46,10 +46,12 @@ LIBMODBUS_LD_AGE=0
 LIBMODBUS_LT_VERSION_INFO=$LIBMODBUS_LD_CURRENT:$LIBMODBUS_LD_REVISION:$LIBMODBUS_LD_AGE
 AC_SUBST(LIBMODBUS_LT_VERSION_INFO)
 
+AC_CANONICAL_HOST
+
 # Check whether we are building for Win32
 os_win32="false"
-case "${target}" in
-      *mingw32)
+case "${host_os}" in
+      *mingw32*)
 		os_win32="true"
 	;;
 esac