lmbench: Fix setting LDLIBS failure

scripts/build is interpreted as an sh script which may not recognize variable
assignment operator += and thus give the following error and fail to append
LDLIBS.

../scripts/build: 21: ../scripts/build: LDLIBS+=-lm: not found

Use the basic assignment instead.

Signed-off-by: He Zhe <zhe.he@windriver.com>
Signed-off-by: Khem Raj <raj.khem@gmail.com>
(cherry picked from commit a6a8fc75ba)
Signed-off-by: Armin Kuster <akuster808@gmail.com>

meta-oe/recipes-benchmark/lmbench/lmbench-3.0-a9/0002-build-Adjust-CFLAGS-LDFLAGS-to-append-values-passed-.patch

@@ -22,7 +22,7 @@ index 34a1371..3786741 100755
  trap 'rm -f ${BASE}$$.s ${BASE}$$.c ${BASE}$$.o ${BASE}$$; exit 1' 1 2 15
  
 -LDLIBS=-lm
-+LDLIBS+=-lm
++LDLIBS="$LDLIBS -lm"
  
  # check for HP-UX's ANSI compiler
  echo "main(int ac, char *av[]) { int i; }" > ${BASE}$$.c