Fix linking of microbenchmark program

On some setups, benchmark_residual linkage fails with an undefined
reference to clock_gettime(). Adding -lrt fixes that.

Patch-from: Ozkan Sezer <sezeroz@gmail.com>

configure.ac

@@ -437,6 +437,11 @@ if test x$enable_stack_smash_protection = "xyes" ; then
 	XIPH_GXX_STACK_PROTECTOR
 	fi
 
+if test "x$sys_linux" = xtrue ; then
+       LIB_CLOCK_GETTIME=-lrt
+       fi
+AC_SUBST(LIB_CLOCK_GETTIME)
+
 AC_CONFIG_FILES([ \
 	Makefile \
 	src/Makefile \

microbench/Makefile.am

@@ -36,3 +36,5 @@ noinst_HEADERS = util.h
 noinst_PROGRAMS = benchmark_residual
 
 benchmark_residual_SOURCES = benchmark_residual.c util.c
+
+benchmark_residual_LDADD = @LIB_CLOCK_GETTIME@