elfutils/flex: Disable parallel make ptest compile

The recent change enabling parallel make for ptest compile/install caused
autobuilder failures in these two recipes. Disable parallel make here
for now until someone can debug the race and get it fixed (preferably
upstream).

(From OE-Core rev: 12755e3e771eb2f1628e2b3dd7138c8766973d82)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/recipes-devtools/elfutils/elfutils_0.187.bb

@@ -56,6 +56,7 @@ do_compile_ptest() {
 	cd ${B}/tests
 	oe_runmake buildtest-TESTS oecheck
 }
+PTEST_PARALLEL_MAKE = ""
 
 do_install_ptest() {
 	if [ ${PTEST_ENABLED} = "1" ]; then

meta/recipes-devtools/flex/flex_2.6.4.bb

@@ -60,6 +60,7 @@ RDEPENDS:${PN}-ptest += "bash gawk make"
 do_compile_ptest() {
 	oe_runmake -C ${B}/tests -f ${B}/tests/Makefile top_builddir=${B} INCLUDES=-I${S}/src buildtests
 }
+PTEST_PARALLEL_MAKE = ""
 
 do_install_ptest() {
 	mkdir -p ${D}${PTEST_PATH}/build-aux/