Remove implicit LDFLAGS, CPPFLAGS, and "ldflags_dirs" build step

The default settings for LDFLAGS and CPPFLAGS were there since the initial commit to ruby-build:

    LDFLAGS="-L${PREFIX_PATH}/lib"
    CPPFLAGS="-I${PREFIX_PATH}/include"

However, it's not clear to me what these settings help with. A typical Ruby installation will initialize files in these directories, but it will do so regardless of the environment variables.

So, let's remove them and see what breaks.