Putting `net-snmp-config --cflags` into CFLAGS causes major pollution;
it overrides optimization and debugging levels, warnings, and more.
While normally we do want to automatically locate library headers,
there has to be a better way to do this. libsnmp is normally in the
usual place and doesn't need special handling. Perhaps people with
libsnmp in a weird place should just need to add the -I flag to their
CFLAGS before build.
Calling out to net-snmp-config --agent-libs causes transitive
dependencies to get pulled in, but we don't need those -- a sensible
dynamic linker pulls those in automatically. Trying to track the
transitive dependencies manually would be a losing battle.
People were recently hitting this on Debian sid/jessie, where libpci
is in the transitive dependency list but isn't otherwise one of our
build dependencies.
