No on likes warnings during configure, so tame autoconf: Provide non-…

…empty 4th arg to AC_CHECK_HEADER

(explicit include prereqs) to only perform compiler test, avoiding older preprocessor test. (Regarding ADL
test.)

Also, follow design pattern used in OpenCL test with a backup variable to avoid tainting CFLAGS.

configure.ac

@@ -203,8 +203,11 @@ scrypt="no"
 if test "$found_opencl" = 1; then
 	if test "x$adl" != xno; then
 		ADL_CPPFLAGS=
+		SAVED_CFLAGS=$CFLAGS
 		CFLAGS="$CFLAGS -I$srcdir"
-		AC_CHECK_HEADER([ADL_SDK/adl_sdk.h], [have_adl=true; ADL_CPPFLAGS=-I$srcdir], [have_adl=false],[])
+		AC_CHECK_HEADER([ADL_SDK/adl_sdk.h], [have_adl=true; ADL_CPPFLAGS=-I$srcdir], [have_adl=false], 
+			[ [#include <stddef.h>] ])
+		CFLAGS=$SAVED_CFLAGS
 		if test x$have_adl+$have_cgminer_sdk = xfalse+true; then
 			AC_CHECK_HEADER([$CGMINER_SDK/include/ADL_SDK/adl_sdk.h], [have_adl=true; ADL_CPPFLAGS=-I$CGMINER_SDK/include], [have_adl=false],[])
 		fi