build: resolve build failure with riemann-client in non-standard location
build: resolve build failure with riemann-client in non-standard location

You are building a C++ program/library, so _CFLAGS is the wrong
variable (see _CXXFLAGS) to use. But since the result from pkg-config
are preprocessor flags anyway, they ought to go into _CPPFLAGS.

bigdatadev/riemannpp,bigdatadev/riemannpp