configure.ac: Refactor Windows backend settings into one occurence

The Windows backend is used both with MinGW and Cygwin, and since most
settings are identical they can be set in one place.

configure.ac

diff --git a/configure.ac b/configure.ac
index 95623f2..ed84e42 100644
--- a/configure.ac
+++ b/configure.ac
@@ -63,29 +63,25 @@
 		[#include <poll.h>])
 	;;
 *-mingw*)
-	AC_DEFINE(OS_WINDOWS, 1, [Windows backend])
-	AC_SUBST(OS_WINDOWS)
 	AC_MSG_RESULT([Windows])
 	backend="windows"
-	PC_LIBS_PRIVATE="-lsetupapi -lole32 -ladvapi32"
-	LTLDFLAGS="${LTLDFLAGS} -avoid-version -Wl,--add-stdcall-alias"
-	AC_CHECK_TOOL(RC, windres, no)
-	AC_DEFINE([POLL_NFDS_TYPE],[unsigned int],[type of second poll() argument])
 	;;
 *-cygwin*)
-	AC_DEFINE(OS_WINDOWS, 1, [Windows backend])
-	AC_SUBST(OS_WINDOWS)
 	AC_MSG_RESULT([Cygwin (using Windows backend)])
 	backend="windows"
 	threads="posix"
-	PC_LIBS_PRIVATE="-lsetupapi -lole32 -ladvapi32"
-	LTLDFLAGS="${LTLDFLAGS} -avoid-version -Wl,--add-stdcall-alias"
-	AC_CHECK_TOOL(RC, windres, no)
-	AC_DEFINE([POLL_NFDS_TYPE],[unsigned int],[type of second poll() argument])
 	;;
 *)
 	AC_MSG_ERROR([unsupported operating system])
 esac
+if test "$backend" = windows; then
+	AC_DEFINE(OS_WINDOWS, 1, [Windows backend])
+	AC_SUBST(OS_WINDOWS)
+	PC_LIBS_PRIVATE="-lsetupapi -lole32 -ladvapi32"
+	LTLDFLAGS="${LTLDFLAGS} -avoid-version -Wl,--add-stdcall-alias"
+	AC_CHECK_TOOL(RC, windres, no)
+	AC_DEFINE([POLL_NFDS_TYPE],[unsigned int],[type of second poll() argument])
+fi
 AC_SUBST(THREAD_CFLAGS)
 AC_SUBST(PC_LIBS_PRIVATE)
 LIBS="${LIBS} ${PC_LIBS_PRIVATE}"