Use __attribute__((format(printf, ...))) directly

Instead of defining the macro _R_(), define __attribute__() as a macro for
compilers that do not support __attribute__(), namely Microsoft Visual
Studio.

Signed-off-by: Bart Van Assche <bvanassche@acm.org>

test-tool/Makefile.am

@@ -1,6 +1,5 @@
 AM_CPPFLAGS=-I. -I${srcdir}/../include \
-	"-D_U_=__attribute__((unused)) " \
-	"-D_R_(A,B)=__attribute__((format(printf,A,B)))"
+	"-D_U_=__attribute__((unused)) "
 AM_CFLAGS=$(WARN_CFLAGS)
 AM_LDFLAGS=-no-undefined
 LIBS = ../lib/libiscsipriv.la