Add appendable flags to Makefile
Add appendable flags to Makefile

Allows user environment CFLAGS/CXXFLAGS to be specified which append to
defaults. Also adds LDFLAGS to the final etc compilation step.

This allows the user to compile and link with Boost easily from the
command line, by using a command like:
`make CXXFLAGS="-DBOOST_SUPPORTED -I/usr/local/opt/boost/include"
LDFLAGS+="-L/usr/local/opt/boost/lib -lboost_system -lboost_filesystem"`

fhanau/Efficient-Compression-Tool,fhanau/Efficient-Compression-Tool