Remove ancient W/A for cygwin.

This W/A was for issue 5.
nickg#5

But src/rt/signal.h was gone.
nickg@b99b6b1

configure.ac

@@ -71,13 +71,6 @@ PKG_CHECK_MODULES([CHECK], [check >= 0.9.4], [], [])
 
 AX_DEFINE_DIR([DATADIR], [datadir/nvc], [Installation data directory])
 
-# On cygwin local headers can sometimes be included instead
-# of system ones which is worked around using -I-. A better
-# solution would be to have automake use -iquote.
-case $host_os in
-  *cygwin* ) CFLAGS="$CFLAGS -I-" ;;
-esac
-
 AC_DEFINE_UNQUOTED([SYSTEM_CC], ["$(which $CC)"], [System compiler])
 
 AC_ARG_ENABLE([native],