nginx-module-opentracing: adjusted libstdc++-devel build requirement.

The libjaegertracing_plugin.so requires libstdc++.a, and that library
is provided by the corresponding package in Fedora and RHEL 8, while
older RHEL 7 does not have it out of the box.

Additionally, Jaeger plugin requires gcc >= 4.9 while RHEL 7 has 4.8,
thus making it effectively impossible to build the plugin without
installing additional software collections from Redhat (e.g. devtoolset-8).

rpm/SPECS/Makefile.module-opentracing

diff --git a/rpm/SPECS/Makefile.module-opentracing b/rpm/SPECS/Makefile.module-opentracing
index 2a2a9f4..679ae7c 100644
--- a/rpm/SPECS/Makefile.module-opentracing
+++ b/rpm/SPECS/Makefile.module-opentracing
@@ -42,7 +42,9 @@
 BuildRequires: cmake
 %define _cmake_cmd %{__cmake}
 %endif
+%if (0%{?rhel} >= 8) || (0%{?fedora} > 0)
 BuildRequires: libstdc++-static
+%endif
 BuildRequires: yaml-cpp-devel
 endef
 export MODULE_DEFINITIONS_opentracing