scons: FreeBSD platform related adjustments

- Use gmake instead of make
- Use different library suffixes
- Different compiler flags
scons: Temporarily disable gssapi support in bind

It has to be fixed and GSSAPI reenabled.
scons: Temporarily disable html support in libxml2

It should be fixed and html support reenabled again.
scons: Temporarily disable agent tests build

Test has to be reenabled, build and ran successfully ASAP.
scons: Temporarily disable -Werror flag
vnsw: Add FreeBSD compiler defines
REBASE THIS: Netlink related incompatibilites
REBASE THIS: FreeBSD vs Linux incompatibilites