Make fails because bin directory is missing

[lckarssen]

Simply running make fails:

g++ -I fvlib -I fvfutil -g -Wall -D_NOT_R_FILEVECTOR  -std=c++0x fvlib/*.cpp fvfutil/text2fvf.cpp fvfutil/convert.cpp -lcppunit -o bin/convert
/usr/bin/ld: cannot open output file bin/convert: No such file or directory
collect2: error: ld returned 1 exit status
Makefile:100: recipe for target 'bin/convert' failed
make: *** [bin/convert] Error 1

Running mkdir bin before running make fixes the problem, so we should add a line to that effect to the Makefile.