of just ${PKGBASE}, e.g. M4DIR=m4>=1.0 will set M4DIR to the installation
prefix of m4 if it matches "m4>=1.0", otherwise it will set M4DIR to
${M4DIR_DEFAULT}.
loop are only used afterwards, split out the EVAL_PREFIX code into a
separate file find-prefix.mk that can be included at any point to
generate a prefix-computation for the variables specified by FIND_PREFIX
at the inclusion point.