Cygwin: Define target_builddir autoconf and Makefile variables

This is now required as cygwin_build is defined in terms of
target_builddir.

(Note that in other subdirectories, the autoconf variable
target_builddir is AC_SUBST-ed as a side-effect of using a macro from
winsup/acinclude.m4, which is perhaps less than ideal)

winsup/testsuite/Makefile.in

@@ -12,6 +12,7 @@ SHELL:=@SHELL@
 srcdir:=@srcdir@
 objdir:=.
 libltp_srcdir=$(srcdir)/libltp
+target_builddir:=@target_builddir@
 
 target_alias:=@target_alias@
 build_alias:=@build_alias@

winsup/testsuite/configure

@@ -587,6 +587,7 @@ PACKAGE_URL='https://cygwin.com'
 ac_unique_file="Makefile.in"
 ac_subst_vars='LTLIBOBJS
 LIBOBJS
+target_builddir
 AR
 CPP
 OBJEXT
@@ -1771,6 +1772,8 @@ ac_compiler_gnu=$ac_cv_c_compiler_gnu
 
 
 
+. ${srcdir}/../configure.cygwin
+
 ac_ext=c
 ac_cpp='$CPP $CPPFLAGS'
 ac_compile='$CC -c $CFLAGS $CPPFLAGS conftest.$ac_ext >&5'
@@ -2790,6 +2793,8 @@ else
 fi
 
 
+
+
 ac_config_files="$ac_config_files Makefile"
 
 cat >confcache <<\_ACEOF

winsup/testsuite/configure.ac

@@ -12,8 +12,12 @@ AC_PREREQ(2.59)
 AC_INIT([Cygwin Testsuite],[0],[cygwin@cygwin.com],[cygwin],[https://cygwin.com])
 AC_CONFIG_SRCDIR(Makefile.in)
 
+. ${srcdir}/../configure.cygwin
+
 AC_PROG_CC
 AC_PROG_CPP
 AC_CHECK_TOOL(AR,ar)
 
+AC_SUBST(target_builddir)
+
 AC_OUTPUT(Makefile)