iscsi-perf: Use gettimeofday if clock_gettime is unavailable

clock_gettime doesn't existing on OS X
(http://stackoverflow.com/questions/5167269/clock-gettime-alternative-in-mac-os-x
) so add an autoconf test to check if the platform has it and fall back
on gettimeofday if not.

Signed-off-by: Sitsofe Wheeler <sitsofe@yahoo.com>

utils/Makefile.am

@@ -1,6 +1,6 @@
-AM_CPPFLAGS=-I../include "-D_U_=__attribute__((unused))" \
+AM_CPPFLAGS = -I../include "-D_U_=__attribute__((unused))" \
 	"-D_R_(A,B)=__attribute__((format(printf,A,B)))"
-AM_CFLAGS=$(WARN_CFLAGS) -lrt
+AM_CFLAGS = $(WARN_CFLAGS)
 LDADD = ../lib/libiscsi.la
 
 bin_PROGRAMS = iscsi-inq iscsi-ls iscsi-perf iscsi-readcapacity16 \