Revert "Don't use the printf command; it's not guaranteed to be there."

This reverts commit 67c5849744.

It's not *guaranteed* to be there in *every* UN*X, but it's in at least
the Single UNIX Specification V3, as well as in shells commonly used in
non-UNIX UN*Xes :-), so it'll be there in the UN*Xes we'll be run on.

Change-Id: I541f7607055a24d6933d10244f85eea60052a3d8
Reviewed-on: https://code.wireshark.org/review/13325
Reviewed-by: Guy Harris <guy@alum.mit.edu>

acinclude.m4

@@ -1804,7 +1804,7 @@ fi
 AC_DEFUN([AC_WIRESHARK_GCC_SYSTEM_INCLUDE],
 [
 	if test "x$GCC" = "xyes" -o "x$CC" = "xclang" ; then
-		$1=`AS_ECHO_N(" $$1") \
+		$1=`printf %s " $$1" \
 			| sed -e 's/  *-I *\// -isystem\//g' -e 's/^ *//'`
 	fi
 ])