Contrib: fixed jaeger-client-cpp building on RHEL/CentOS 7.

contrib/src/jaeger-client-cpp/Makefile

diff --git a/contrib/src/jaeger-client-cpp/Makefile b/contrib/src/jaeger-client-cpp/Makefile
index f83773e..1b4dd4e 100644
--- a/contrib/src/jaeger-client-cpp/Makefile
+++ b/contrib/src/jaeger-client-cpp/Makefile
@@ -13,7 +13,7 @@
 jaeger-client-cpp: jaeger-client-cpp-$(JAEGER_CLIENT_CPP_VERSION).tar.gz .sum-jaeger-client-cpp
 	$(UNPACK)
 	$(APPLY) $(SRC)/jaeger-client-cpp/CMakeLists.txt.patch
-ifeq ($(shell rpm --version 2>&1 || echo FAIL),)
+ifeq ($(shell rpm --version >/dev/null 2>&1 || echo FAIL),)
 ifeq ($(shell test `rpm --eval '0%{?rhel}'` -gt 0 -a `rpm --eval '0%{?rhel}'` -lt 8 && echo 1),1)
 	$(APPLY) $(SRC)/jaeger-client-cpp/CMakeLists.txt.suppress-gcc-version-check.patch
 endif