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.
build: fail when riemann-client.pc is not found

It makes no sense to proceed beyond PKG_CHECK_MODULES in the absence
of riemann-client.pc, because the project is wholly dependent on it
to build libriemannpp.
Update .gitignore
build: resolve build failure of the client program

src/client/riemannpp.cpp uses riemann_new_client(), so _must_
link against libriemann-client, too.
Merged build fixes from Jan Engelhardt