Allow the user to set compile options
Allow the user to set compile options

The makefile now checks for three environment variables:

 - LWTNN_CXX_FLAGS: extra compiler options
 - LWTNN_LDFLAGS: extra linker flags
 - LWTNN_LIBS: extra libraries

Hopefully this makes it easier to avoid hacking the makefile in custom
environments.

jwsmithers/lwtnn,lwtnn/lwtnn,lwtnn/lwtnn,jwsmithers/lwtnn,lwtnn/lwtnn,jwsmithers/lwtnn