Suppose you have the following makefile:
unit.exe: unit.o
g++ unit.o -o unit.exe
unit.o: unit.cc unit.h
g++ -c unit.cc -o unit.o
In order to modify it to use an external library you have to use the GCC -I and -L options:
unit.exe: unit.o
g++ unit.o -o unit.exe -L /c/cryptopp -l ws2_32 -l cryptopp
unit.o: unit.cc unit.h
g++ -I /c/cryptopp -c unit.cc -o unit.o
Often a makefile would contain a variable that is passed to the compiler and a variable that is passed to the linker, for example CFLAGS and LDFLAGS. If that is the case, then it might be easier to add the "-I" and "L" options to the compiler and linker variables.
See also here for a way to comiple CryptoPP.