check for missing COMSPEC variable in cygwin.

Cygwin unsets the COMSPEC environment variable on some installations.
This is due to an error reported first time in 2005, but not solved.
There are no exact pattern when this happens, in this case it happened
on 2 windows 8.1 VM installations (one private one azure).

added check in configure.ac to alert the user earlier

COMSPEC is used in sal/osl to start processes and as such vital, and in 
some perl scripts, therefore hardcoding to e.g. cmd32.exe (or the power
shell) is not an option.

Limited check to work only for cygwin.

Change-Id: I52bb6667c52560ed1a239ac301811605b8cddcbf
Reviewed-on: https://gerrit.libreoffice.org/20255Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Michael Meeks <michael.meeks@collabora.com>
Tested-by: Michael Meeks <michael.meeks@collabora.com>

configure.ac

@@ -282,6 +282,10 @@ if test "$build_os" = "cygwin"; then
     PathFormat "$BUILDDIR"
     BUILDDIR="$formatted_path"
     x_Cygwin=
+    AC_MSG_CHECKING(for explicit COMSPEC)
+    if test -z "$COMSPEC"; then
+        AC_MSG_ERROR([COMSPEC not set in Environment, please set it an rerun])
+    fi
 fi
 
 AC_SUBST(SRC_ROOT)