do not set STD_* to the empty string

configure.in

@@ -13,7 +13,7 @@ dnl PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS
 dnl ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS
 dnl SOFTWARE.
 
-AC_REVISION($Revision: 1.108 $)
+AC_REVISION($Revision: 1.109 $)
 
 AC_PREREQ(2.13)
 
@@ -26,9 +26,6 @@ AC_PROG_MAKE_SET
 AC_PROG_RANLIB
 AC_PROG_INSTALL
 
-STD_CINCLUDES=""
-STD_CDEFINES=""
-STD_CWARNINGS=""
 AC_SUBST(STD_CINCLUDES)
 AC_SUBST(STD_CDEFINES)
 AC_SUBST(STD_CWARNINGS)
@@ -192,8 +189,8 @@ case "$host" in
 			lib1="-L$pkg/lib -Wl,-R$pkg/lib"
 			lib2="-lpthread -lm -lgcc -lpthread"
 			LIBS="$lib1 $lib2 $LIBS"
-			CPPFLAGS="-I$pkg/include $CPPFLAGS"
-			STD_CINCLUDES="-I$pkg/include"
+			CPPFLAGS="$CPPFLAGS -I$pkg/include"
+			STD_CINCLUDES="$STD_CINCLUDES -I$pkg/include"
 			;;
 		no+yes)
 			AC_MSG_RESULT(PTL2)