Resolve error compiling on Linux (Ubuntu 13.10)

When building on Ubuntu 13.10, I got an error that looks like this, running `autogen.sh`

    Makefile.am:22: error: cgminer_CPPFLAGS must be set with '=' before using '+='
    Makefile.am:13: warning: 'INCLUDES' is the old name for 'AM_CPPFLAGS' (or '*_CPPFLAGS')
    Makefile.am: installing './depcomp'
    autoreconf: automake failed with exit status: 1

This patch fixes that. I don't know if it's the "correct" solution, but I was able to get the program compiled and run it.