configure.ac: don't undefine scoped variable (#594)

* configure.ac: don't undefine scoped variable

To get this script to run with Autoconf 2.71 on macOS I had to remove the undefine of the backend for loop variable. It seems scoped to the for loop and also isn't referenced later in the script so it seems OK to remove it.

* configure.ac: remove cygwin specific CFLAGS #598

Notes:
Remove cygwin specific Win32 CFLAGS and treat the build like a posix build

Credit:
Will Cosgrove, Brian Inglis

configure.ac

@@ -36,12 +36,9 @@ case "$host" in
     CFLAGS="$CFLAGS -DLIBSSH2_WIN32"
     LIBS="$LIBS -lws2_32"
     ;;
-	*-cygwin)
-	CFLAGS="$CFLAGS -DLIBSSH2_WIN32"
+    *darwin*)
+    CFLAGS="$CFLAGS -DLIBSSH2_DARWIN"
     ;;
-	*darwin*)
-	CFLAGS="$CFLAGS -DLIBSSH2_DARWIN"
-	;;
     *hpux*)
     ;;
     *osf*)
@@ -128,8 +125,6 @@ fi
 m4_set_foreach([crypto_backends], [backend],
   [AM_CONDITIONAL(m4_toupper(backend), test "$found_crypto" = "backend")]
 )
-m4_undefine([backend])
-
 
 # libz